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Interlocking

Interlocking is a safety mechanism in railway signaling systems designed to prevent conflicting train movements through complex track arrangements, such as junctions, crossovers, and sidings, by ensuring that signals, points (switches), and other apparatus operate in a coordinated and locked sequence.^[1] This system enforces predefined logical conditions to prohibit hazardous operations, like setting a route over an occupied track or moving points under a passing train, thereby minimizing the risk of collisions and derailments.^[2] The concept of interlocking originated in the mid-19th century amid the rapid expansion of rail networks, which increased the frequency of accidents at busy intersections.^[3] The first mechanical interlocking frame, using levers and physical locks to interconnect signals and switches, was developed by John Saxby in England in 1856, marking a pivotal advancement in train control.^[4] By the late 1800s, similar systems were adopted in the United States and Europe, with early implementations focusing on manual operation from centralized towers to manage traffic at key locations.^[5] Over time, interlocking technology has evolved through several generations to enhance reliability, scalability, and efficiency in increasingly dense rail corridors. Mechanical systems, reliant on rods, wires, and bolts for physical enforcement, gave way to electro-mechanical relay-based interlockings in the early 20th century, which used electrical circuits and relays for more compact and remote control.^[6] Post-World War II innovations introduced solid-state interlocking (SSI) in the 1980s, leveraging semiconductor technology for faster processing and reduced maintenance, followed by computer-based interlocking (CBI) systems from the 1990s onward, which employ software algorithms and digital interfaces for programmable logic and integration with advanced signaling like ETCS (European Train Control System).^[7] These modern variants support higher train densities and automation, as seen in high-speed rail networks.^[3] Today, interlocking remains a cornerstone of railway safety worldwide, mandated by international standards such as those from the International Union of Railways (UIC) to verify track conditions before authorizing movements.^[8] Its implementation in interlocking areas—typically spanning stations, yards, and crossings—ensures fail-safe operations, with redundancy features like vital relays or software verification to guard against failures.^[9] As rail systems integrate with IoT and AI for predictive maintenance, future interlockings are poised to incorporate machine learning for dynamic route optimization while upholding uncompromised safety.^[6]

Introduction

Definition and Purpose

Interlocking in railway signaling refers to an arrangement of signal apparatus, including signals, switches, and other appliances, that prevents conflicting train movements through junctions, crossovers, and stations.^[10] This system ensures that routes are set and locked in a manner that prohibits simultaneous operation of signals permitting opposing or overlapping train paths on the same track section.^[11] The primary purpose of interlocking is to enforce fail-safe conditions, where physical or logical constraints—such as mechanical locks or electronic circuits—block the setup of unsafe routes, thereby reducing collision risks arising from human error or equipment failure.^[10] By integrating with overall signaling systems, interlocking permits signals to display only permissible aspects, such as "proceed," when the intended route is verified as clear and secure.^[11] This fail-safe design defaults to a safe state during any system fault, preventing the authorization of dangerous movements.^[2] Key safety benefits include the elimination of route conflicts, which safeguards train operations at complex track arrangements, and protection for workers on or near tracks by ensuring no movements occur into occupied sections detected via track circuits or similar means.^[10] These features collectively minimize derailments, collisions, and other hazards, enhancing overall railway reliability.^[12] Interlocking originated as a response to early rail accidents caused by errors in manual signaling practices, introducing automated constraints to promote safer routing.^[13] Implementations range from mechanical levers to electronic logic systems, each upholding these core safety principles.^[10]

Basic Components

Interlocking systems in railways rely on a set of core components to ensure safe train routing through junctions and crossovers, preventing conflicts by coordinating movements. These fundamental elements include signals, which authorize train movements; points or switches, which direct trains onto specific tracks; detection devices such as track circuits or axle counters, which monitor track occupancy; and locking devices, which enforce constraints on operations.^[14]^[10] Signals serve as visual or cab-based indicators that control train progression, with main signals governing regular movements over routes and shunting signals permitting low-speed maneuvers within yards. Points, also known as switches, consist of movable rails that diverge or converge tracks, typically operated by electric or pneumatic machines to set routes accurately. Detection systems, including track circuits that use electrical continuity through the rails to sense train presence via axle-induced short circuits, or axle counters that tally entering and exiting axles to determine section occupancy, provide real-time confirmation of track clearance. Locking devices, ranging from mechanical bolts and clamps in traditional setups to algorithmic constraints in digital systems, secure points and signals to maintain route integrity.^[14]^[10] Interconnection methods link these components to enforce mutual exclusivity among routes, utilizing physical linkages like rods and wires in mechanical systems or electrical and electronic circuits in modern installations that transmit control signals and feedback. For instance, circuit controllers operated by switch points or locking mechanisms select signal control paths, ensuring no conflicting route can be established simultaneously. Track circuits play a critical role in detection by verifying route clearance up to the clearance point—typically located 2–6 meters beyond the fouling point to account for vehicle overhang—before authorizing a signal to display a proceed aspect, thereby activating route locking to protect the path until the train passes.^[14]^[10]^[15] Control interfaces allow operators to initiate route selections while incorporating built-in safeguards against invalid configurations, such as lever frames with mechanical interlocks in older systems, control panels with push-button arrays, or visual display unit (VDU) screens in computer-based interlockings that require sequential confirmation of point positions and track occupancy. These interfaces ensure that only feasible routes, verified by detection and locking, can be set, maintaining the system's safety principles.^[14]

Historical Development

19th-Century Origins

The rapid expansion of railway networks in the United Kingdom and the United States during the 1840s and 1850s brought increased traffic volumes, but rudimentary manual signaling systems—relying on flags, lamps, and verbal communications—proved inadequate, leading to frequent collisions and derailments. For instance, in the UK, a series of head-on collisions, such as the 1854 Croydon accident where two trains smashed into each other due to misinterpreted hand signals, underscored the risks of uncoordinated operations at junctions and on single tracks. Similarly, in the US, incidents like the 1853 Norwalk wreck, caused by a train passing a stop signal on a drawbridge, resulted in 48 deaths and highlighted the dangers of inconsistent signaling practices amid growing freight and passenger demands. These events prompted regulatory scrutiny and calls for mechanical safeguards to prevent conflicting train movements.^[16] The concept of interlocking emerged in the mid-19th century as a mechanical solution to enforce safe routing by physically linking signal levers and points (switches). In 1856, British engineer John Saxby patented the first practical system, featuring a lever frame where rods and cams interlocked to ensure that signals could only be cleared after points were set correctly, preventing setups for collisions. This innovation addressed the flaws in independent manual controls by making unsafe combinations impossible without force. In the US, early adoption of similar mechanical interlocks occurred in the 1870s, with the first installation—a Saxby & Farmer frame—operational in 1870 at Trenton Junction, New Jersey, on the Pennsylvania Railroad, marking the importation of British technology to busy American crossings. American inventors soon contributed, as seen in the 1875 Spuyten Duyvil installation by J.M. Toucey and William Buchanan, which used a domestic mechanical frame to interlock points and signals at a complex New York junction.^[17]^[18]^[19] By the mid-1870s, mechanical interlocking spread to key UK sites. Patents facilitated this diffusion, such as Saxby's foundational UK design and subsequent US adaptations licensed through firms like Saxby & Farmer. Despite these advances, early systems remained fully manual, operated by levermen in wooden signal boxes, and were vulnerable to deliberate overrides or mechanical jams under heavy use, limitations that would later drive the shift toward electrical enhancements in the 20th century.^[20]^[21]

20th-Century Advancements

The early 20th century marked a significant shift in railway interlocking from purely mechanical systems to electro-mechanical designs, enabling greater scalability and remote operation. In the United States, companies like General Railway Signal (GRS), founded in 1904, pioneered the adoption of electric levers and solenoids that integrated electrical controls with mechanical components, allowing operators to manage switches and signals from centralized panels. By the 1910s, GRS systems were widely installed, such as the electric interlocking machine at various U.S. railroads, which replaced manual lever operations with powered mechanisms to handle complex junctions more efficiently.^[22] This transition improved reliability by reducing physical wear on mechanical parts and facilitating larger interlockings that could accommodate growing rail traffic.^[23] The relay era, spanning the 1920s to 1950s, further advanced interlocking through the use of vital relays—electrically robust devices designed to ensure fail-safe logic circuits for signal and switch control. These relays formed the core of all-relay interlockings, where circuits enforced geographic locking (preventing conflicting movements within defined areas) or route locking (securing specific paths).^[24] A notable example was the New York Central Railroad's installations in the 1930s, including the all-relay plant at Harmon, New York, completed in 1932, which utilized over 1,000 relays to manage high-volume electrified operations with enhanced safety. By the 1940s and 1950s, vital relays had become standard for creating complex interlocking logic, supporting centralized traffic control and reducing human error in dense rail networks.^[25] World War II accelerated the standardization of signaling and interlocking systems in both Europe and the United States to meet urgent demands for efficient troop and supply transport. In the U.S., the Military Railway Service deployed railroad experts to upgrade European networks, installing standardized relay-based interlockings and communications to boost capacity amid wartime disruptions.^[26] European railways, heavily impacted by bombing and occupation, adopted unified power signaling practices post-1945 to rebuild interoperability, drawing on pre-war relay technologies for quicker restoration.^[27] This period emphasized vital circuit designs to minimize failures under high-stress conditions, laying groundwork for post-war reliability improvements.^[28] Key milestones in this era included the United Kingdom's 1930 adoption of power signaling regulations, which formalized electrical operation of signals and points under the Southern Railway's standards, promoting consistent electro-mechanical practices across networks.^[29] In the U.S., the Association of American Railroads (AAR) established comprehensive standards for relay interlockings in the 1950s, including nomenclature and circuit symbols in their 1956 manual, which ensured uniform design and maintenance for safety-critical systems nationwide.^[30] These developments solidified relay-based interlocking as a scalable solution, influencing global practices through the mid-century.^[31]

21st-Century Innovations

The 21st century marked a profound digital transformation in railway interlocking systems, shifting from relay-based designs to solid-state and computer-based architectures that enhanced reliability, reduced maintenance, and enabled integration with broader traffic management networks. Solid-state interlockers (SSI), initially prototyped by Westinghouse and others in the late 20th century, saw widespread adoption across Europe starting in the 2000s, with deployments accelerating in the 2010s to replace aging electromechanical systems. For instance, the UK's network extensively utilized SSI for its modular, processor-driven logic, which minimized wiring and space while supporting remote diagnostics, leading to widespread installations.^[32]^[33] A key innovation was the integration of interlocking with communication-based train control (CBTC) systems, which allowed dynamic, moving-block operations to increase capacity on urban networks. In 2005, New York City Transit initiated trials on the L line, deploying CBTC to overlay existing fixed-block interlocks with radio-based positioning and continuous supervision, reducing headways from 2.5 to 1.5 minutes and enabling automatic train operation.^[34] Similarly, the European Rail Traffic Management System (ERTMS), particularly ETCS Levels 2 and 3, incorporated advanced interlocking during 2010s rollouts across Denmark, Norway, Sweden, and the Netherlands, eliminating lineside signals in favor of in-cab displays and GSM-R communications for seamless cross-border interoperability.^[35] These systems standardized interlocking interfaces via initiatives like EULYNX, through modular trackside components.^[35] In Asia, China's Chinese Train Control System (CTCS), deployed since 2008 on high-speed rail lines, exemplified global scaling of digital interlocking, supporting speeds up to 350 km/h with quasi-moving-block functionality and centralized supervision. CTCS-3, the core variant for premier routes like Beijing-Shanghai, integrates interlocking logic with balise-based positioning and radio transmission, ensuring collision avoidance across a network exceeding 40,000 km by 2025.^[36] Recent advancements by 2025 have incorporated AI for predictive maintenance in interlocking, such as Siemens' Signaling X platform, which uses machine learning to analyze sensor data from switches and signals.^[37] Blockchain technology has emerged for secure remote software updates, as demonstrated in prototypes like the Blockchain-based Railway Control System (BRCS), where consensus mechanisms ensure tamper-proof modifications to interlocking logic across distributed nodes, enhancing cybersecurity without central vulnerabilities.^[38] Emerging security measures address quantum computing threats, with quantum-resistant encryption integrated into interlocking communications; for example, Nokia's post-quantum cryptography trials for rail networks employ lattice-based algorithms to protect signaling data against future attacks, maintaining integrity in ERTMS environments.^[39] Sustainability features, including energy-efficient processors in solid-state designs, have reduced power consumption in interlocking compared to relays, supporting greener operations through low-voltage microcontrollers and optimized logic that minimize idle-state draw.^[33] These innovations collectively enable higher capacity, resilience, and environmental compliance in global rail networks.

Principles of Operation

Configuration and Implementation

The design of railway interlocking systems begins with assessing the layout's operational requirements, typically employing either route-based or speed-based configurations to ensure safe train movements. In route-based designs, the system defines predefined paths (routes) through the network, locking signals, points, and other elements to prevent conflicts along each route; compatibility between routes is predetermined, often by engineering experts using tabular specifications that divide the station into discrete paths.^[40] Speed-based configurations, in contrast, focus on imposing speed restrictions at junctions or crossovers rather than fixed routes, allowing more flexible train routing while maintaining safety through aspect indications that limit velocity based on track occupancy and geometry.^[41] Plant diagrams, schematic representations of the track layout, illustrate these interdependencies by mapping signal placements relative to points and track circuits, enabling visualization of how a signal clearance locks conflicting routes.^[42] Implementation involves a structured sequence of steps to integrate the interlocking into the railway infrastructure. Initial site surveys evaluate junction complexity, including track counts, traffic density, and environmental factors, to determine the system's scope and hardware needs.^[43] Simulation testing follows, using software tools like RailSys to model train movements, route setting, and failure scenarios, verifying logic before physical installation; for instance, Simulink-based models can execute test scenarios up to 18 times faster than hardware emulation for stations with thousands of equations.^[44] Commissioning concludes the process with proof-of-performance tests, which systematically validate functional responses (e.g., route requests) and safety constraints (e.g., no overlapping routes) through on-site inspections, insulation proving, and conformance to design drawings.^[45]^[46] Common use cases highlight the practical application of these systems in complex environments. In station throats—the densely interlocked areas leading to platforms with multiple converging tracks—route-based interlocking manages simultaneous arrivals and departures, preventing conflicts across ladders of switches; for example, at high-traffic terminals, it coordinates dozens of routes to optimize throughput without compromising safety.^[47] Level crossing protections integrate railway signals with highway traffic controls, using interlocking logic to synchronize barriers, flashing lights, and road signals; radar-based obstacle detection ensures the crossing clears before train signals proceed, as implemented in systems like the Vital Harmon Logic Controller for multiple UK crossings.^[48] Scalability is achieved through modular designs that facilitate expansion and upgrades with minimal disruption. These systems employ standardized interfaces and networked components, such as Ethernet-based vital communications, allowing incremental addition of modules for new tracks or stations; Network Rail's modular signalling initiative, launched post-2010, exemplifies this with interchangeable hardware like Alstom's Smartlock, reducing trackside needs while supporting ERTMS integration across varying network densities.^[49]^[50] Track circuits, as basic occupancy detectors, underpin these configurations by providing real-time input to the interlocking logic.^[48]

Locking Mechanisms

Locking mechanisms in railway interlocking systems enforce constraints to prevent conflicting train movements by securing signals, points, and routes in defined positions. These mechanisms operate across mechanical, electrical, and software-based forms, each tailored to ensure fail-safe operation. Mechanical locking, for instance, uses physical devices such as clamp locks on signal levers or points to physically prevent unauthorized movements; a clamp point lock secures the switch tongue via a mechanical clamp and slide rod connection, providing reliable fixation without relying on electrical power or electronic controls.^[51] Electrical locking employs circuit supervision to monitor and control vital circuits, ensuring that locks remain engaged unless specific safe conditions are verified, such as through forced-drop relays that de-energize to a safe state upon failure and track circuits detecting shunts as low as 0.06 ohms to confirm route clearance.^[52] Software-based locking utilizes state machines to manage resource allocation, where finite state machines model track and route states (e.g., free, locked, active) with guards enforcing mutual exclusion, preventing incompatible routes from activating simultaneously.^[53] Key concepts in locking include route locking, which secures an entire route—including points and opposing signals—once a train passes a cleared signal and the signal returns to stop, holding until the train fully clears the route to avoid conflicts.^[54] Sectional release, a variant of route locking, allows partial unlocking of rear sections as the train progresses, releasing locks on cleared track circuits in sequence while maintaining protection for forward sections, thus optimizing capacity without compromising safety.^[54] Approach locking preemptively secures the route upon displaying a proceed aspect, preventing alterations (e.g., point movements or route cancellation) until the approaching train either passes the signal or a timed release expires after the signal normalizes to stop, typically based on track occupation over a defined distance.^[54]^[52] Fail-safe principles underpin all locking mechanisms, designed to default to a restrictive state upon any fault. In vital systems, one-out-of-two voting (or 2-out-of-2 architectures) ensures that agreement between dual channels is required for permissive actions, with disagreement causing de-energization to safe; this is extended to 2-out-of-3 voting for enhanced fault tolerance, where a single channel failure does not inhibit safety.^[55] Hardware diversity, incorporating varied processors or software versions across channels, mitigates common-mode failures by reducing the likelihood of simultaneous faults from shared vulnerabilities.^[56] The core logic for mutual exclusion can be expressed formally; for incompatible routes A and B, activation of route A (requiring signal A clear and point 1 set) implies negation of route B (signal B blocked and point 2 unset):

(\text{Signal}_A \land \text{Point}_1) \rightarrow \lnot (\text{Signal}_B \lor \text{Point}_2)

This ensures no overlapping permissions, verifiable through state invariants in software models.^[53]

Types of Interlocking

Mechanical Interlocking

Mechanical interlocking systems utilize physical lever frames to control railway points and signals, ensuring that conflicting movements cannot occur simultaneously. These frames typically consist of a bank of levers mounted in a wooden or metal structure, with each lever connected via wire runs or rods to distant apparatus up to several hundred meters away. The interlocking mechanism within the frame incorporates tumblers—rotating bars or plates—and scotch blocks, which are sliding or pivoting components that physically block unauthorized lever movements. Tumblers, as seen in designs like those of the Midland Railway, engage with notches on adjacent levers to enforce dependencies, while scotch blocks provide additional locking for specific functions such as facing point locks. Wire runs, often double-wired for signals and single-rodded for points, transmit the mechanical force from lever pulls to operate the equipment, with compensation devices to account for thermal expansion.^[57]^[58] In operation, sequential lever movements are mandated by the physical layout of notches and tappets in the frame's locking tray, where pulling one lever releases or blocks others based on predefined routes. For instance, a signal lever cannot be cleared until associated point levers are positioned correctly, as the tumbler or scotch block obstructs its path until the prerequisite levers are reversed. Detection of point and lock positions occurs through mechanical contacts, such as pull rods or detectors that engage circuit controllers or physical locks, confirming alignment before allowing further operations. This fail-safe design relies entirely on gravity and mechanical resistance, with levers returning to a "normal" position if not held, preventing incomplete setups.^[57]^[8] The primary advantages of mechanical interlocking include inherent fail-safety, as physical barriers make it impossible to set up hazardous conditions without breaking the system, providing high reliability for simple layouts. However, limitations are significant: these systems are often confined to installations with fewer than 100 levers due to the escalating complexity and space required for interconnections in larger plants, and they demand regular, labor-intensive maintenance to prevent wear on wires, rods, and locking elements. Modern applications are largely confined to preservation efforts on heritage railways, where operational examples educate on historical practices; a notable instance is the 1870s-style lever frame at York's National Railway Museum, restored and functional for simulated train workings as of 2025.^[8]^[59]^[60]

Electro-Mechanical Interlocking

Electro-mechanical interlocking represents a hybrid approach that builds upon the mechanical base of traditional lever frames by incorporating electrical components to enhance scalability and control in larger railway installations. These systems integrate electric point machines, which use solenoids to drive switch movements, with mechanical frames that provide physical locking between levers. Solenoids, acting as electromagnetic actuators, allow for remote or powered operation of points and signals, reducing the physical effort required compared to all-mechanical setups while maintaining mechanical safeguards against conflicting routes.^[61] Free-wire systems further support signal control by routing dedicated electrical circuits directly from the interlocking frame to field devices without intermediate relays, enabling straightforward energization of signals based on lever positions and track occupancy. This design facilitated the management of complex junctions where purely mechanical systems became impractical due to size and force limitations.^[62] In operation, throwing a lever in the electro-mechanical frame mechanically interlocks with other levers to prevent unsafe combinations, while simultaneously energizing electrical circuits to activate solenoids in point machines or signal mechanisms. Feedback loops are integral, with position indicators—often lights or contacts on the switches—confirming that points have fully aligned before releasing locks or clearing signals, ensuring no movement proceeds until verification. For instance, an indicating lock on the lever remains engaged until electrical confirmation of switch position is received, preventing premature signal clearance. These systems typically operate on low-voltage DC power, with circuit controllers on levers breaking or making contacts to route power to specific functions, providing both local control and basic remote monitoring capabilities.^[61]^[63] Historical examples illustrate the adoption of electro-mechanical interlocking in the early 20th century. In the United Kingdom during the 1920s, Westinghouse's Style E system exemplified this hybrid technology, combining mechanical lever frames with electric locks and solenoid-driven points for efficient control in busy terminals. In the United States, Union Switch & Signal installations in the 1940s, such as the rebuilt electro-mechanical plant at St. Louis Union Station following a 1940 fire, demonstrated resilience and adaptability, using 66-lever frames with electric point operations to handle high-volume passenger traffic. These setups proved reliable, with the St. Louis installation processing thousands of daily movements post-restoration. By the 1980s, electro-mechanical interlocking had largely been phased out in favor of all-relay systems, which offered greater flexibility and reduced maintenance without mechanical wear. However, remnants persist in low-traffic areas, such as rural branches or heritage lines, where the hybrid reliability suffices without justifying full modernization; for example, some North American sidings retain these installations for basic protection.^[64]

Relay Interlocking

Relay interlocking systems utilize all-relay logic composed of electromagnetic switches to enforce safe train routing decisions at junctions and crossovers. These systems evolved from electro-mechanical precursors by eliminating mechanical levers and relying solely on relay panels for control. Vital relays, designed with fail-safe principles such as non-weldable contacts and independent magnetic circuits, handle safety-critical functions like route locking, while non-vital relays manage auxiliary tasks such as indications and monitoring.^[65]^[66] Design philosophies in relay interlocking differ between geographical and route-based (free-wired) approaches. In the geographical method, relays are organized by track layout location, facilitating modular updates for specific areas without widespread rewiring. The route-based approach, conversely, wires relays according to logical route combinations, prioritizing flexibility in complex yards but increasing maintenance complexity. Both employ rack-mounted panels housing thousands of relays, with vital circuits using closed-circuit principles where de-energization defaults to a safe state.^[67]^[68]^[69] Operationally, relay trees form interconnected circuits that implement Boolean logic for route selection, where input conditions from track circuits and point detectors energize specific relays to authorize signals only for non-conflicting paths. For multi-aspect signals, line locking—via stick or approach relays—ensures routes remain secured against occupation until a train passes, preventing premature clearing of subsequent aspects. This logic is documented through ladder diagrams translating control tables into relay energization sequences.^[70]^[71]^[8]^[69] Relay interlocking offers proven fail-safety through its closed-circuit vital design, where any break or failure de-energizes relays to block unsafe routes, and supports capacities exceeding 200 routes per installation. Post-2010 advancements include integration with LED signal indicators for direct drive compatibility, enhancing visibility and reducing power needs while maintaining relay logic integrity. Notable examples encompass 1960s Japanese developments in geographical relay systems for high-density networks and ongoing 2025 upgrades in India, such as the Route Relay Interlocking at Karjat Central Cabin, which modernized signaling for improved reliability.^[72]^[73]^[74]^[75]^[76]^[77]^[78]^[79]

Electronic Interlocking

Electronic interlocking systems represent a microprocessor-based evolution in railway signalling, utilizing solid-state logic to manage route setting, signal control, and point operations without mechanical or relay components. These systems employ dual central processing units (CPUs) in a redundant configuration, often following a double 2-out-of-2 voting architecture, where both CPUs execute identical safety-critical algorithms and cross-verify outputs to ensure fail-safe performance and high availability even during peak traffic.^[80] Interfaces integrate directly with supervisory control and data acquisition (SCADA) platforms for real-time monitoring and the European Train Control System (ETCS) for seamless train-to-infrastructure communication, enabling centralized oversight of large-scale networks.^[81] Operationally, these systems rely on sophisticated software algorithms for dynamic routing, which allocate train paths adaptively based on traffic demands, occupancy data, and predictive analytics to prevent conflicts. Self-diagnostics are facilitated through built-in test equipment that performs continuous integrity checks on hardware and logic, automatically isolating faults and triggering fail-safe modes to maintain operational continuity.^[82] This programmable approach allows for modular updates to interlocking logic, supporting complex topologies while adhering to deterministic execution for safety. Key advantages include exceptional capacity, with systems capable of handling over 1,000 routes in dense urban or high-speed corridors, alongside remote diagnostics that reduce maintenance interventions and downtime.^[83] However, challenges persist in rigorous software validation to achieve CENELEC EN 50129 SIL4 certification, the highest safety integrity level, which demands exhaustive formal verification and lifecycle management to mitigate risks from coding errors or environmental factors.^[84] Cybersecurity has become a focal concern, with modern designs incorporating encrypted communications, intrusion detection, and zero-trust architectures to counter evolving threats like ransomware targeting signalling infrastructure. Interoperability is enhanced through standards such as EULYNX Baseline 4, which standardizes interfaces for cross-vendor integration across European networks.^[85] Recent developments in the 2020s emphasize cloud-hybrid architectures, blending on-site processors with remote cloud resources for scalable processing and predictive maintenance, as demonstrated in Alstom's 2024 deployment of digital interlocking for South London's signalling upgrade, which improved coverage and resilience via IP-based transmission.^[81] Experimental initiatives in the EU, including trials by the German Aerospace Center (DLR), are investigating quantum computing for optimizing rail transport operations, including route planning and conflict resolution in timetables, with prototypes showing potential for faster solutions to complex routing problems by 2025.^[86]

Terminology and Standards

U.S. Terminology: Complete vs. Incomplete

In U.S. railroad practice, interlocking plants are sometimes described as complete or incomplete based on the scope of route protection provided. A complete interlocking ensures full safeguarding of all possible routes and movements within the plant, including mainlines, crossovers, derails, and yard tracks, preventing conflicting operations through interconnected signal appliances and switches. In contrast, an incomplete interlocking provides protection only for primary routes, such as mainline through tracks, while secondary movements—like those involving sidings or auxiliary yards—rely on manual overrides, hand-thrown switches, or operator supervision to avoid conflicts. This partial approach allows for cost-effective implementation in lower-traffic areas but requires additional procedural safeguards for unprotected routes.^[87] The terminology emerged in the 1910s amid the Pennsylvania Railroad's (PRR) efforts to standardize signaling across its network, where detailed interlocking specifications were developed to address growing complexity in urban and junction operations. PRR installations during this period, such as those at Sunnyside Yard and Penn Station approaches in 1910, exemplified early applications of these classifications to balance safety with operational demands.^[88] Historical examples of incomplete interlockings include several Chicago-area plants from the 1930s, where high-density freight and passenger traffic led to hybrid systems protecting main routes while leaving yard crossovers under manual control, as seen in facilities like the 16th Street Bridge and Harlem Avenue setups.^[89] Today, incomplete interlockings are uncommon in new constructions, particularly for high-speed rail corridors. Federal Railroad Administration (FRA) regulations under 49 CFR Part 236 require Positive Train Control (PTC) integration on lines with passenger speeds ≥80 mph or freight speeds ≥79 mph where mandated, to mitigate risks through enforced route protection and enhanced collision prevention.^[90]

International Variations and Standards

In Europe and the United Kingdom, railway interlocking systems are characterized by an emphasis on power signaling principles, where signals default to a "danger" aspect and are only cleared when a safe route is verified through interlocking logic. The Solid State Interlocking (SSI) system, pioneered in the 1980s by British Rail, represents a foundational microprocessor-based approach that computes and enforces route settings, point locking, and signal controls.^[91]^[92] For software components in these systems, the European standard EN 50128 specifies requirements for the development lifecycle of software used in railway control and protection systems, mandating high safety integrity levels (SIL 1-4) to mitigate failure risks in signaling applications.^[93] In Asia, Japan integrates interlocking functions with Automatic Train Control (ATC) systems to enhance operational safety on dense networks, particularly high-speed Shinkansen lines. The SAINT (Shinkansen ATP and Interlocking) system, developed by Hitachi in the early 2000s, combines automatic train protection (ATP) with centralized interlocking logic executed on shared digital platforms, enabling real-time route supervision and fault-tolerant operations across multi-station layouts.^[94] This integration supports Japan's technical regulatory standards, which require interlocking devices to prevent collisions through fail-safe mechanisms aligned with international norms like IEC 62278.^[95] In China, railway signaling and interlocking adhere to the TB/T series of industry standards, such as TB/T 3027 for computer-based interlocking systems and TB/T 10436 for engineering inspections, ensuring compatibility with the country's extensive high-speed network expansions.^[96]^[97] Australia features regional variations in route locking practices due to state-specific railway management, with standards emphasizing flexible adaptations for diverse terrains. For instance, the Australian Rail Track Corporation (ARTC) mandates sequential release of approach locking via track circuits to secure routes, while New South Wales guidelines under Transport for NSW detail overlap protections to prevent conflicts in signaling overlaps beyond fouling points.^[98]^[99] These variants, outlined in AS 7711 Signalling Principles, allow for customized interlocking in freight-heavy corridors.^[54] In African contexts, particularly low-electrification regions like parts of Sub-Saharan Africa, railway development strategies prioritize sustainable infrastructure to accommodate unreliable power grids, as outlined in broader UIC initiatives for the region.^[100] The International Union of Railways (UIC) provides guidelines through initiatives like the African Railway Green Deal, promoting sustainable mobility and carbon neutrality by 2050 to boost network resilience.^[101] Global harmonization efforts are advanced by the UIC's International Railway Standards (IRS), which establish core principles for interoperable signaling and interlocking, including topological modeling under IRS 30100 to standardize route definitions across borders.^[102]^[103] The Institution of Railway Signal Engineers (IRSE) supports this through resources on universal safety principles, fostering consistency in interlocking design.^[104] In 2025, UIC launched its Climate Change Adaptation and Resilience Group to address resilience in railway infrastructure, with ongoing projects focusing on hazards like flooding and heatwaves.^[105]^[106]

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Accident Archive
Displaying 1 to 10 of 9213 accidents. Filtered for any year. 1803 1804 1805 1806 1807 1808 1809 1810Missing: collisions signaling
[16]
[PDF] RAILWAY SIGNAL BOXES: A REVIEW | Historic England
the invention of John Saxby who made a significant advance in mechanical interlocking between points and signals for which he obtained a patent in 1856. He ...
[17]
https://historicengland.org.uk/research/results/reports/6071/RailwaySignalBoxes_AReview
[18]
[PDF] Union Junction Interlocking Tower HAER NO. MD-50 (Northeast ...
The first interlocking machine in the United States was installed in 1870 in Trenton, New Jersey on the Camden and Amboy Division of the PRR. 3. Saxby ...Missing: UK | Show results with:UK
[19]
Saxby and Farmer - Graces Guide
Feb 25, 2020 · Company established in Haywards Heath by John Saxby who was the first to achieve interlocking of route selection and signalling.
[20]
Railway 200: Signalling - Rail Engineer
Apr 29, 2025 · Mechanically interlocked signal boxes are still in use and, for example, the Monks Sidings signal box interlocking and frame installed in 1875 ...
[21]
GRS - History | PDF | Rail Transport - Scribd
Rating 5.0 (1) GRS mechanical interlocking assembly department in 1910. Signal Company offered railroads the Johnson type of mechanical interlocking. When the Company was
[22]
Rail Innovation Timeline – 1900s
Mar 24, 2022 · The transition from a fully mechanical interlocking, through electro-mechanical to fully electrical interlocking took place in the US around ...
[23]
Relay - Railroad Signals of the U.S.
The relay forms part of a simple (basically) three element circuit, consisting of the relay, a battery, and the track.Missing: 1950s | Show results with:1950s
[24]
Introduction To North American Railway Signaling 2008928725 ...
Types of Relay Vital signal relays seem to have been developed largely as a result of the track circuit, invented in 1872. By 1900, relays seem to have ...<|control11|><|separator|>
[25]
Railroaders in Olive Drab: The Military Railway Service in WWII
American railroaders installed modern communications equipment to coordinate the increased train movements. They also added 100 miles of double track to ...<|control11|><|separator|>
[26]
The Impact of WWII on European Rail Networks - DDay.Center
World War II changed European rail networks forever. The war made railways carry soldiers, supplies, and civilians all over a continent under siege.
[27]
[PDF] Railroad Signals - Okonite
This section will outline the basic components of a rail signal system and highlight typical cable requirements for each device. Today's signaling system ...
[28]
Southern Railway Train Signalling Regulations 1930
Train signalling regulations, Southern Railway, "Standard Regulations for Train Signalling on Double and Single Lines, Including Description of Signalling ...
[29]
[PDF] Symbols, Aspects and Indications, 1956 - chartertoconductor
In railway signaling, standard symbols are the work of the Signal Section, Association of American Railroads. The symbols, comprising 24 sheets, were ...
[30]
[PDF] RAILWAY SIGNALS, SIGNS, MARKS & MARKERS 2nd Edition ...
Power Signalling). News Letter. September. _. ud. Signals. Melbourne: Victoria Railways. Australia, Western Australia. 1974. General Appendix to the Book ...
[31]
[PDF] Signalling market study Final Report | ORR
Nov 9, 2021 · Solid State Interlocking (“SSI”). SSI was developed by British Railways (“BR”),. Westinghouse and GEC-General Signal (“GEC”) as part of a ...
[32]
Forty years of Solid State Interlocking - Rail Engineer
Oct 28, 2025 · This year marked 40 years since the commissioning of the first conventional signalling Solid State Interlocking (SSI) at Leamington Spa on 8 ...Missing: Europe 2000s 2010s
[33]
Controlling and Executing Communications Based Train Control ...
Oct 27, 2008 · New York City Transit (NYCT) has undertaken a program to install CBTC (Communications Based Train Control) technology.Missing: Subway trials
[34]
Evolving from ETCS to ERTMS - Global Railway Review
Nov 15, 2022 · Michel Ruesen of the ERTMS Users Group, explores ERTMS in Europe and the ways that the deployment has evolved over time.
[35]
[PDF] China's High-Speed Rail Development - World Bank Document
An instrumented train is run every 10 days to check condition. A four-hour window is provided every night for maintenance. The sixth chapter explains the ...
[36]
Siemens Mobility presents Signaling X and next-level Rail Services ...
Sep 23, 2024 · We are proud to launch our brand-new suite Signaling X to lead rail signaling and control systems into the digital future.
[37]
[PDF] Scaling a Blockchain-based Railway Control System Prototype for ...
Mar 11, 2021 · The blockchain-based railway control system aims to be more resilient, integrated, and cost-efficient, automating traffic management and ...
[38]
[PDF] IS YOUR RAILWAY STILL SECURE IN THE QUANTUM ERA? - Nokia
The advent of quantum computing, unfortunately, may render many of today's most popular public key encryption algorithms, such as Diffie-Hellman and. Rivest- ...Missing: resistant | Show results with:resistant
[39]
[PDF] Decision making in railway interlocking systems based ... - AIMS Press
Sep 15, 2023 · Classical railway interlocking systems are route-based and the compatibility of routes is decided (usually by human experts) in advance [7] ( ...
[40]
Route signalling vs. speed signalling - an oversimplification? - RMweb
May 17, 2023 · Route signalling indicates the destination, while speed signalling indicates the speed over a junction, not the route. Both convey routing and ...Missing: configurations | Show results with:configurations
[41]
Efficient verification of railway infrastructure designs against ...
Jun 15, 2017 · Most interlocking specifications use a route-based tabular approach, which means that a train station is divided into possible routes, which are ...
[42]
[PDF] Verification of Railway Interlocking Systems and Optimisation of ...
Over the years, the number of trains, the number of tracks, the complexity of networks increase and are still increasing. Directing trains on efficient routes, ...
[43]
Validation process for railway interlocking systems - ScienceDirect
Oct 15, 2016 · In this paper we show how the problem has been addressed by a manufacturer at the final validation stage of production interlocking systems.Missing: RailSys commissioning
[44]
(PDF) Verification of railway interlocking systems - ResearchGate
Aug 9, 2025 · In this paper, we explain how we built an executable model in NuSMV of a railway interlocking based on the application data.
[45]
[PDF] PR S 47113 - Inspection and Testing of Signalling - Transport for NSW
Mar 18, 2025 · Inspections and tests shall verify detailed conformance to the particular vital signalling design drawings, compliance with the applicable ...
[46]
Regional Rail Proof of Concept — TransitMatters
The pinch point in the South Station throat is an interlocking called Tower 1. It features a complex of switches called a ladder track: trains from tracks ...
[47]
Applying logic to level crossings - Rail Engineer
Aug 12, 2015 · An interlocking is required to correctly and safely control the protecting railways signals, road traffic light signals and lifting barriers.Missing: throat | Show results with:throat
[48]
Alstoms Smartlock equipment at the heart of modular signalling ...
Mar 4, 2011 · The modular signalling concept is an initiative launched by Network Rail to develop low-cost, efficient and easy-to-install technology that can be used to re- ...
[49]
[PDF] Report of a comparative analysis of the Interlocking Systems ... - Disit
Apr 19, 2013 · The development of SSI was undertaken jointly by British Rail, GEC Genera1 Signal Company Limited and. Westinghouse Signals LTD in the United ...Missing: deployment | Show results with:deployment
[50]
Focus on clamp point locks: Mechanical safety in modern turnouts
Oct 28, 2025 · Its mechanical connection via clamp and slide rod enables reliable locking without electrical supply or electronic interlocking. Clamp point ...<|separator|>
[51]
[PDF] Signal & Train Control Compliance Manual
This rule subjects automatic block signal systems, traffic control systems, interlockings, automatic train stop, train control, and cab signal systems or ...
[52]
[PDF] Towards the verification of a generic interlocking logic: Dafny ... - arXiv
Feb 29, 2024 · Interlocking logic is a mutual exclusion protocol for railway systems, controlling traffic within stations. This paper uses Dafny to verify a ...
[53]
[PDF] Signalling Principles | RISSB's
Interlocking functions include sectional route locking, point locking, release (e.g. for ground frames) locking, route locking and approach locking. In the ...
[54]
[PDF] Fault Tolerant Fail Safe System for Railway Signalling1 - IAENG
Hence for railway signaling application, fault tolerant architecture with 2-out-of-3 voting, instead of. 3-out-of-4 according to Byzantine fail-safe principle, ...
[55]
[PDF] Reliability and Security Analysis on 3-vote-2 Voting System - NADIA
Jul 4, 2013 · But for some upgraded 3-vote-2 voting systems, three computers respectively fix three sets of interlocking software with diverse versions ...
[56]
[IRFCA] Indian Railways FAQ - Train Working Systems – Interlocking
Solid State Interlocking (SSI) is similar to RRI, but the interlocking is achieved by electronic circuitry instead of electromechanical relay systems. They ...Missing: 2010s | Show results with:2010s
[57]
Midland Railway lever frames - The Signal Box
The entire locking frame was built above operating floor level, with the lever catches and interlocking mechanism all encased in a black metal surrounding.Missing: mechanical scotch blocks
[58]
[PDF] Mechanical and Electro-Mechanical Interlocking - chartertoconductor
The electric machine consists of cabinet, frame, levers, indication magnets, lever locks, lever lights, tappets, locking, terminal board and rotary circuit ...Missing: clamp | Show results with:clamp
[59]
Network Rail team provides TLC for a museum icon
May 23, 2017 · The lever frame and instruments were linked to a computer, allowing the frame to be worked in a way that demonstrated how a train would be ...
[60]
None
### Summary of Trenton Electro-Mechanical Interlocking
[61]
None
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[62]
[PDF] Chapter-19-Electric-Interlocking-1950.pdf - chartertoconductor
In addition to being mechanically operated, this contact block is under the control of two sets of solenoid magnets, so that should the switch not complete its ...
[63]
Union Station retires 90-year-old interlocking signal system - Metrolinx
Nov 13, 2019 · Union Station retires 90-year-old interlocking signal system. Check out the system of handle-pulls, audible clicks and banks of early 20th century technology.Missing: remnants | Show results with:remnants
[64]
[PDF] Introduction and Overview to Interlockings Course 106
Course 106 covers an overview of interlockings, basic terminology, regulations, design, types, and functions.
[65]
[PDF] Chapter 21: Relay & Electronic Interlocking Section 1 - Indian Railway
Calling On signal shall detect all the points including interlocked level crossings in the route, which the main signal above it detects, except points in the ...
[66]
https://indianrailways.gov.in/railwayboard/uploads/directorate/signal/2023/21-Relay%2520%2526%2520Electronic%2520Interlocking.pdf
[67]
[PDF] Geographical vs. Functional modelling by statecharts of interlocking ...
Actually, in the geographical approach, in case of a change to some parts of the interlocking system there may be some modules that do not need to be changed ...Missing: armature philosophy
[68]
[PDF] Safety and Design of Signal Circuits - jonroma.net
This type of stick circuit can be adapted to any type of signal control and signal mech- anism. Stick circuits are becoming more important with the introduction.
[69]
FAULT TREE ANALYSIS OF ROUTE RELAY INTERLOCKING ...
Boolean Equation for SMCR is: SMCR = SM's KEY Refer Figure 2 for Operating ... Selection is done by the Operation of Individual LR Relay for the Signaled Route.
[70]
[PDF] Analysis of Relay Interlocking Systems via SMT-based Model ...
Abstract—Relay Interlocking Systems (RIS) are analog elec- tromechanical networks traditionally applied in the safety-critical domain of railway signaling.
[71]
Fail safe relays for interlocking system for rails - Intertech Rail
Fail-safe relays are designed to ensure that the interlocking system operates safely even in the event of a failure.Missing: closed- | Show results with:closed-
[72]
Range of SIL4 safety critical relay (vital) - CLEARSY
The fail-safe guarantee is obtained by the fact that the moving part of the relay is orientated in relation to earth's attraction. This way, via the effect of ...
[73]
[PDF] route relay interlocking (siemens)
All the required drawings of a two-line station with Siemens Route Relay Interlocking System are given in this book. They include signaling plan, Route section ...Missing: capacity | Show results with:capacity
[74]
[PDF] Safety, availability and capacity of electronic interlocking - WIT Press
Fourteen stations equipped with DSI electronic interlocking systems, controlling up to 170 open-line installations and allowed up to 230 routes, were soon set ...<|separator|>
[75]
[PDF] Design of Microlok II Interlockings - jonroma.net
Jun 25, 2010 · Direct drive of LED signal lights is permitted. The interlocking design must provide a method to permit disconnection as per the Microlok ...Missing: post- | Show results with:post-
[76]
https://www.jonroma.net/media/signaling/standards/world/australia/SCP23.pdf
[77]
WO2005113315A1 - Railway signalling system and interlocking
So-called "geographical" relay based interlockings were developed in the 1960s in order to resolve some of the problems with free wired systems. A large ...
[78]
Central Railway Commissions Modern Route Relay Interlocking with ...
Oct 13, 2025 · This upgrade enhances signalling precision, minimizes human intervention, and ensures faster train operations. Engineering & Electrical Works.
[79]
All electronic computer interlocking system based on double 2-vote-2
Aug 7, 2025 · The double 2-vote-2 interlocking computer is composed of three parts which are two sets of the interlocking mainframes, the redundancy optical ...
[80]
Alstom commissions new South London signalling system
Nov 7, 2024 · In Alstom's digital interlocking the technology transmits train control information digitally, via the IP network, providing enhanced coverage.Missing: electronic deployments
[81]
[PDF] electronic interlocking - CSE IIT KGP
1.2 Advantages of Electronic Interlocking System: (a) System can be tested at factory level using simulation panels. (b) Non-Interlocking period is less ( ...Missing: challenges | Show results with:challenges
[82]
(PDF) Overview of the Full Electronic Interlocking System
This paper describes the development process of the system and manufacturers, and then introduces the research status of the interlocking system.
[83]
ENCE V4: new Electronic Interlocking System
Mar 9, 2020 · ... SIL-4 level according to the CENELEC EN 50129 standard. The Electronic Interlocking is divided into three subsystems: the hardware, software ...
[84]
Managing the standardization of Electronic Interlocking (EI) devices ...
Jul 9, 2025 · EULYNX Baseline 4 Release 4 Specifications (2025): Defines harmonized functional and technical interfaces for electronic interlocking systems.
[85]
Fewer delays, better route planning and timetables: quantum ...
How can highly complex planning tasks in rail transport be solved better and faster than before with the help of quantum computers?Missing: interlocking EU
[86]
The Association of American Railroads (AAR)
Your trusted source for news, policy positions and insights from North America's freight rail industry.Data Center · About AAR · Contact Us · News & EventsMissing: incomplete | Show results with:incomplete
[87]
[PDF] signal and train control regulations, technical applications, and ...
A traffic control system is a block signal system under which train movements are authorized by block signals or cab signals whose indications supersede the ...
[88]
None
### Summary of Pennsylvania Railroad Interlocking Standards in the 1910s
[89]
INTERLOCKING TOWERS - Chicago Railfan.com
Separate interlocking later replaced with new consolidated interlocking at Polk Street. 16TH STREET BRIDGE Electric interlocking installed 1930. STILL IN USE.Missing: incomplete | Show results with:incomplete
[90]
49 CFR Part 236 -- Rules, Standards, and Instructions Governing the ...
Track circuits and route locking shall be provided and shall be effective when the first pair of wheels of a locomotive or a car passes a point not more than 13 ...
[91]
Signalling a fresh, new approach - Rail Engineer
May 2, 2023 · The cost of signalling in the UK rail industry has been a concern for many years and signalling renewals are a major cost for Network Rail at around £200 ...
[92]
DIGITAL SIGNALLING AND CONTROL - Modern Railways
May 24, 2018 · This article provides an introduction to the contemporary control and signalling technologies and how they can contribute to the continuing digitisation of the ...Missing: sustainability | Show results with:sustainability
[93]
[PDF] Signalling Systems - Critical Software
EN 50128 - Railway applications - Communication, signalling and processing systems - Software for railway control and protection systems. EN 50129 - Railway ...Missing: UK power
[94]
[PDF] SAINT Integrated Signaling System with High Reliability and Safety
Mar 4, 2008 · This paper describes the development of SAINT. (Shinkansen ATP and interlocking system) shown schematically in Fig. 1, in response to the needs ...
[95]
[PDF] Interlocking System Based on Concept of Securing a Train ...
Railway Bureau, Ministry of Land, Infrastructure, Transport and Tourism Technical. Regulatory Standards on Japanese Railways. IEC62278:2002. Railway ...
[96]
Product--Traffic control technology
The system complies with international standards including IEC 62280 to ensure global compatibility; and complies with domestic standards including TB/T 3027- ...
[97]
Chinese Industry Standard: TB, TB/T, TBT -- 50
Oct 19, 2025 · TB/T 10184-2021. TB/T 10436-2021, (Railway Computer Interlocking Engineering Inspection Regulations (with instructions)), TB/T 10436-2021. TB/T ...Missing: China | Show results with:China
[98]
[PDF] Common Signal Design Principles S1 - ARTC - Extranet
Oct 13, 2015 · The approach locking shall be released by the normal passage of the train over the first and second track circuits in sequence immediately past ...
[99]
[PDF] Signalling Design Principles - Overlaps - Transport Standards
Jan 30, 2022 · This Principle addresses the requirements for locking out opposing routes leading into or situated within an overlap by a particular route of ...
[100]
Developing a New Technical Strategy for Rail Infrastructure in Low ...
This research reviewed the literature on the current state of rail infrastructure to identify development areas that have the greatest potential for increasing ...Missing: adaptations | Show results with:adaptations
[101]
Sustainable Railways Africa - UIC - International union of railways
Sep 22, 2021 · As a continuation of the railway climate commitment declared by UIC in 2019 and by means of this 'African railway green deal for sustainable ...Missing: interlocking low
[102]
IRS | UIC - International union of railways
Jun 14, 2018 · They are the outcome of independent work conducted by the railway operators in order to harmonise the railways in an efficient and realistic way ...
[103]
[PDF] IRS 30100: RailTopoModel - Railway infrastructure topological model
Sep 1, 2016 · The International Railway Standards (IRS) are structured in a General Part and in some eventual Application Parts. The General Part is valid ...
[104]
(PDF) Railway Signalling Principles - ResearchGate
Now, after restoring the signal, approach locking will hold the route locked. ... signal is not part of the block system but controlled by an interlocking route.<|control11|><|separator|>
[105]
UIC launches Climate Change Adaptation and Resilience Group
May 12, 2025 · UIC has launched a railway resilience group, following the successful completion of the first two projects of the RERA (Resilient Railways) family.Missing: interlock housings
[106]
Developing railway solutions that can withstand the effects of climate ...
Apr 16, 2025 · Alstom has been at the forefront of developing innovative solutions for railway infrastructure to withstand floods, thermal stress, and harsh winters.