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Trailing wheel

A trailing wheel, also known as a trailing axle, is an unpowered wheelset on a steam locomotive positioned behind the driving wheels to support the weight of the firebox and cab, thereby improving stability and enabling larger boiler and firebox designs for increased steam production.^[1]^[2] These wheels, typically arranged in a trailing truck for multi-axle configurations, also help guide the locomotive through curves and distribute weight more evenly across the rails.^[1]^[2] In the Whyte notation system, developed by Frederick Methvan Whyte for classifying steam locomotives based on wheel arrangements, the number of trailing wheels is denoted by the final digit; for instance, a 4-6-2 configuration indicates four leading wheels, six driving wheels, and two trailing wheels.^[1] Trailing wheels became essential as locomotives evolved in the late 19th and early 20th centuries to handle heavier loads and longer distances, accommodating expansive fireboxes that extended over the rear driving wheels.^[1]^[2] Notable examples include the 4-6-2 Pacific type, introduced around 1901 by Baldwin Locomotive Works for the New Zealand Railways and widely adopted for passenger service due to its balance of speed and power.^[1] The 2-8-4 Berkshire, first built in 1925 by Lima Locomotive Works for the Boston & Albany Railroad (a subsidiary of the New York Central Railroad), featured four trailing wheels to support its large firebox for heavy freight hauling.^[1] Similarly, the 4-8-4 Northern, debuted in 1926 for the Northern Pacific Railway, utilized four trailing wheels for enhanced stability in high-speed passenger and freight operations across varied terrain.^[1] In some designs, trailing wheels were equipped with booster engines to provide additional starting traction for heavy trains.^[2]

Definition and Overview

Definition

A trailing wheel on a steam locomotive is an unpowered wheel or axle positioned behind the driving wheels, typically as part of a trailing truck assembly.^[2]^[1] These wheels primarily serve to support the rear section of the locomotive's frame, bearing the weight of the cab, firebox, and related components without providing any tractive effort for propulsion.^[2]^[1] Unlike driving wheels, which are powered by the locomotive's cylinders to transmit force to the rails and move the train, trailing wheels contribute solely to load distribution and stability. They also differ from leading wheels, which are located at the front to guide the locomotive around curves and reduce wear on the forward driving wheels.^[2]^[1] In the Whyte notation system for classifying wheel arrangements, trailing wheels are denoted by the final numeral, indicating their count.^[1]

Role in Locomotive Design

Trailing wheels are integral to the structural and functional architecture of steam locomotives, primarily by facilitating improved weight distribution. Positioned behind the driving wheels, they bear the substantial load of the enlarged firebox and cab, which would otherwise impose excessive pressure on the driving wheels responsible for traction. This redistribution ensures that a significant portion of the locomotive's total weight is supported by the trailing assembly, alleviating frame stress and promoting balanced adhesion across all axles for more effective propulsion.^[3]^[4] In terms of operational stability, trailing wheels guide the rear of the locomotive, particularly at higher speeds, reducing the risk of derailment on curved sections of track. By providing lateral support and preventing the rear from deviating, they minimize oscillations and enhance overall tracking precision, which is critical for safe navigation through varying rail geometries. This guiding function complements the locomotive's forward momentum, contributing to smoother rides and greater reliability in dynamic conditions.^[3]^[2] Additionally, the rear support offered by trailing wheels enables a lower boiler placement, optimizing the center of gravity for the entire assembly. With the firebox positioned more rearward and supported adequately, designers could achieve a reduced height profile, which lowers the gravitational center relative to the rails and improves both stability and efficiency in steam generation. This arrangement allows for enhanced combustion within the larger firebox while maintaining a favorable balance that supports efficient heat transfer and power output.^[3]^[5]

History

Early Development

The trailing wheel first appeared in steam locomotives during the early 1830s as a simple rigid axle positioned behind the driving wheels to provide basic rear support for the firebox and cab, enhancing stability without adding complexity to the powered components. These early implementations were typically unpowered and fixed rigidly to the frame, allowing for modest weight distribution on uneven tracks common to nascent railway networks. The design emerged as engineers sought to balance the growing size of boilers and fireboxes in freight-hauling engines, where unsupported rear ends risked instability during operation.^[6] By the mid-1830s, trailing wheels gained widespread adoption in rigid-frame locomotives across Britain and Europe, particularly for short-haul freight and passenger services on relatively straight routes. Key early examples include the Patentee class of 2-2-2 locomotives built by Robert Stephenson and Company in 1833, which added a trailing axle to the earlier Planet design; the three 0-4-2 goods engines built by Robert Stephenson and Company in 1834 for the Stanhope and Tyne Railway, which utilized a single trailing axle to support coal-hauling duties on the line's undulating terrain.^[7] Similarly, in 1834, George Forrester and Company introduced the Swiftsure class of 2-2-2 single-driver locomotives for British railways, incorporating trailing wheels to aid in rear weight distribution during express runs. On the continent, the 1838 Saxonia locomotive for the Leipzig-Dresden Railway in Germany featured a trailing axle in its 0-4-2 arrangement, improving handling for early passenger services. These configurations, often employing rigid pedestal axles as precursors to more advanced swiveling designs, proliferated through the 1840s and 1850s in minimal setups—typically one trailing axle—for operations on lines like the Liverpool and Manchester Railway extensions.^[8] Despite their initial success in supporting basic locomotive needs, rigid trailing wheels began to decline in favor by the late 1800s as railway demands evolved toward longer, curvier tracks and heavier trains. The fixed nature of these axles limited flexibility, causing excessive flange contact and derailment risks on bends, as the rigid wheelbase could not accommodate lateral shifts required for sharper curves. This inflexibility became particularly problematic with the expansion of networks in the 1860s and 1870s, prompting engineers to shift toward articulated bogies and leading trucks that offered greater adaptability, effectively sidelining simple rigid trailing axles in mainstream designs.^[9]^[10]

Reintroduction and Evolution

The reintroduction of trailing wheels in American locomotives occurred between 1890 and 1895, primarily through the development of the 4-4-2 "Atlantic" type, which incorporated a rigid pedestal axle design to support the trailing wheel. This configuration allowed for lowered boiler placement, increasing steam production efficiency by accommodating larger fireboxes without compromising stability. The first such locomotive was built by the Baldwin Locomotive Works in 1894 for the Atlantic Coast Line Railroad, marking a significant advancement in passenger service capabilities.^[11] By around 1900, trailing wheel designs evolved to include radial-stay trucks, which featured pivoting mechanisms for enhanced lateral movement and better negotiation of curves compared to the earlier rigid setups. This improvement addressed limitations in track flexibility, enabling smoother operation on winding routes while maintaining support for expanded firebox areas. The radial truck became a standard feature in subsequent locomotive types, such as the 4-6-2 Pacific and 2-8-2 Mikado.^[11] Around 1915, one-piece cast-steel frames were introduced for trailing trucks, providing greater structural integrity to accommodate booster engines on the trailing axle for additional tractive effort. This innovation supported heavier loads by enhancing durability and reducing maintenance needs.^[12] In 1921, the Delta trailing truck was developed by the Commonwealth Steel Company, featuring an inverted-rocker centering device that improved stability and load distribution through a three-point suspension system. This design was later adapted into four- and six-wheel variants to handle the demands of heavy freight service, allowing for wider fireboxes and increased boiler capacity in larger locomotives.^[13]

Design Features

Mechanical Components

The trailing wheels of a steam locomotive typically consist of two parallel axles, each fitted with a pair of flanged wheels, mounted on a pivoting truck frame that supports the rear portion of the locomotive. This assembly is connected to the main locomotive frame at the rear via equalizer bars, which distribute the weight evenly across the axles and driving wheels to maintain stability on uneven track.^[14]^[15] The truck frame itself is a rigid structure, often cast or fabricated from steel, designed to allow swiveling motion for curve negotiation while carrying the substantial weight of the firebox overhang.^[14] Key mechanical components include pedestals, which are rigidly attached to the truck frame and guide the axle boxes at a slight angle—typically on a 6-foot radius—to promote radial alignment of the axles and prevent excessive lateral play. Suspension is provided by helical or leaf springs mounted in yokes above the axle boxes, with capacities around 6,600 pounds per spring, absorbing vertical shocks and ensuring smooth weight transfer. Bearings, commonly bronze or roller types fitted to 8-inch diameter axle journals (14 inches long), handle both vertical loads up to 18,500 pounds per end and lateral forces, maintaining pressures below 170 pounds per square inch to minimize wear.^[14]^[3] In integration, the trailing axles are positioned parallel to the driving wheel axles but lack coupling rods, permitting independent free rotation to accommodate the firebox's extension over the rear. Equalizer bars—often two in number—link the trailing truck springs to the rear driving springs, forming a system that equalizes loads (e.g., 17,275 pounds per intermediate equalizer) and supports the firebox weight without transmitting tractive effort. This setup enhances overall locomotive stability by allowing the truck to pivot via a spherical bearing at the frame connection.^[14]^[15]

Variations and Innovations

One notable variation in trailing wheel designs involved the integration of booster engines on trailing trucks, which provided auxiliary steam power to enhance starting tractive effort, particularly useful in yard operations or when initiating heavy train movements. These boosters, typically small two-cylinder engines mounted on the rear axle of the trailing truck, were widely adopted on American steam locomotives during the 1920s through 1940s, such as the New York Central's J-class 4-6-4 Hudsons and Chesapeake & Ohio's J-3 class 4-8-4s, adding up to 14,000 pounds of tractive effort without significantly altering the main locomotive configuration.^[16]^[17]^[18] The Delta truck represented another key innovation, featuring a triangular frame structure that improved weight distribution across the wheels and enhanced performance on curved tracks by allowing greater lateral movement. This design was particularly suited for supporting enlarged fireboxes on high-power rigid-frame locomotives and saw extensive use, including 4-8-4 Northerns and various Mikados, where it replaced earlier rigid truck configurations for better stability under load.^[19]^[20] Multi-axle expansions of trailing trucks progressed from the standard two-axle setup to four- or six-axle variants to accommodate the immense weight of oversized fireboxes in heavy-haul steam engines. A prominent example is the 2-6-6-6 "Allegheny" arrangement developed for the Chesapeake & Ohio Railway in the 1940s, where a six-wheel trailing truck with cast-steel framing supported a firebox exceeding 100 square feet in grate area, enabling the locomotive to haul trains of up to 14,000 tons over steep grades while maintaining structural integrity.^[21]^[22] This evolution, building on early 20th-century cast-steel frame advancements, allowed for greater boiler capacity without compromising rail adhesion.^[23]

Applications

In Steam Locomotives

In steam locomotives, trailing wheels primarily served to support oversized fireboxes, enabling greater fuel capacity and higher steam output for both passenger and freight operations. By positioning these non-driving wheels behind the main drivers, designers could extend the firebox rearward without interfering with the powered axles, allowing for larger combustion chambers that burned more coal or oil efficiently. This configuration was essential for generating the sustained power required in demanding service, as seen in the 4-6-2 Pacific type, where the two trailing wheels bore significant weight from the enlarged firebox to boost evaporative capacity.^[4] Similarly, in freight designs like the 2-8-2 Mikado, the trailing wheels supported the tender, which had a capacity of up to 17.5 short tons (15.9 t) of coal and 10,000 US gallons (38,000 L) of water, facilitating prolonged hauls without frequent stops. Operationally, trailing wheels enhanced stability during high-speed passenger runs and heavy freight hauling, especially on uneven or curved tracks common in expansive rail networks. The added support distributed the locomotive's rear weight more evenly, reducing the risk of derailment and improving riding quality at speeds exceeding 70 mph (110 km/h), as demonstrated by Pacific-class engines pulling nearly 1,000-ton trains.^[4] In freight applications, this stability was crucial for maintaining control under heavy loads, with the trailing wheels helping to guide the locomotive through irregularities in the track while the firebox provided consistent steam production.^[24] For example, on the Louisville & Nashville #152, the trailing wheels measured 49 inches (1.24 m) in diameter, were spaced 7.5 feet (2.3 m) from the rear drivers, and carried about 35,300 pounds (16,000 kg), directly aiding balance and preventing excessive stress on the frame.^[25] Trailing wheels were more prevalent in North American steam locomotive designs, where long-haul requirements necessitated larger fireboxes for extended operations across vast distances, compared to the more compact European models that prioritized maneuverability in denser networks with tighter curves. While more prevalent in North America, trailing wheels were also used in European designs, such as the British 4-6-2 Pacifics for express passenger service. This regional emphasis reflected the broader scale of American railroading, with trailing wheels appearing first in U.S. locomotives in 1886 to accommodate growing power demands. In practice, such as on lines like the Soo Line or Southern Railway, they enabled efficient service on challenging terrains, underscoring their role in adapting steam technology to specific operational contexts.^[4]

Wheel Arrangements

Trailing wheels are classified within standard locomotive wheel notation systems, which describe the overall arrangement of wheels on a steam locomotive to indicate its configuration for specific duties. The most widely used system in North America and the United Kingdom is the Whyte notation, developed by Frederick Methvan Whyte in 1901, where the sequence of numbers separated by hyphens represents the number of leading wheels (unpowered wheels at the front for stability), driving wheels (powered wheels providing traction), and trailing wheels (unpowered wheels at the rear). In this system, trailing wheels are denoted by the final numeral; for instance, the 4-6-2 Pacific arrangement features four leading wheels, six driving wheels, and two trailing wheels, while the 2-8-2 Mikado has two leading wheels, eight driving wheels, and two trailing wheels.^[26]^[1] Common wheel arrangements incorporating trailing wheels typically feature up to two or four such wheels in rigid-frame locomotives, though articulated designs can extend to six trailing wheels to accommodate larger boilers and increased weight distribution. Representative examples include the 4-8-4 Northern type, which employs four trailing wheels to support an extended firebox, commonly used in high-speed passenger service, and the 2-6-6-6 Allegheny, an articulated locomotive with six trailing wheels on the rear engine unit for heavy freight hauling over mountainous terrain. These arrangements reflect adaptations for operational needs, with trailing wheels generally limited to a maximum of six across all configurations to maintain balance and track stability.^[22]^[26] In the UIC (International Union of Railways) classification system, prevalent in Europe, wheel arrangements are denoted using a combination of numerals for unpowered axles and letters (A=1 axle, B=2, C=3, etc.) for powered axles, with trailing axles indicated by a numeral following the driving notation. For example, the Whyte 4-6-2 Pacific corresponds to 2'C1' in UIC notation, signifying a two-axle leading bogie (2), three-axle driving bogie (C for six wheels), and one-axle trailing bogie (1 for two wheels). This system highlights differences in trailing axle configurations between freight and passenger locomotives; freight types like the 2-8-2 Mikado (1'C1') often use minimal trailing axles for better adhesion on heavy loads, whereas passenger configurations such as the 4-8-4 Northern (2'D2') incorporate additional trailing axles for enhanced stability at speed.^[26]

Advantages and Limitations

Benefits

Trailing wheels enable the design of larger fireboxes on steam locomotives by supporting the extended rear structure, allowing for greater coal or wood consumption and increased steam generation capacity without overburdening the driving wheels. This enhancement in boiler efficiency directly boosts overall power output, as seen in the 2-8-4 Berkshire class where the four-wheel trailing truck facilitated a wider firebox with expanded heating surface area, resulting in higher horsepower at sustained speeds.^[27] Similarly, in the 2-10-2 Santa Fe type, the trailing truck permitted a larger firebox than earlier designs, improving combustion and power delivery for heavy freight service.^[28] By bearing the weight of the firebox and cab, trailing wheels improve locomotive stability and ride quality, particularly through enhanced rear guidance that minimizes oscillation during operation at speeds exceeding 50 mph. This configuration distributes weight more evenly across the locomotive, improving stability on curves and during reverse travel on steep grades.^[2]^[28] When equipped with a booster engine, trailing wheels further enhance tractive effort by powering the rear axle, providing an additional 20–30% starting power in low-speed scenarios such as yard switching or heavy starts on inclines. For instance, on Mikado-type locomotives, boosters increased tractive effort by approximately 23%, enabling the handling of 392 extra tons of train weight.^[29]^[30]

Drawbacks

The incorporation of trailing wheels, typically mounted on pivoting trucks, adds significant mechanical complexity to steam locomotive designs through components such as center plates, bolsters, springs, equalizers, and safety chains, all of which must be regularly inspected and maintained to federal standards for safe operation. These elements require precise alignment, with center plates fitting to allow at least 3/4 inch extension and limiting lost motion to no more than 1/2 inch, while springs and rigging must be maintained in safe and suitable condition for service.^[31] This elevated parts count contributes to higher maintenance demands and repair costs, as any misalignment or failure in the truck assembly can compromise stability and lead to operational downtime.^[31] The additional mass from trailing wheels and their supporting structure imposes weight penalties that can diminish locomotive efficiency, especially on lighter rail infrastructure or tight curves common in certain regions. By distributing weight away from the driving wheels, trailing trucks reduce the proportion available for traction—a significant portion of the locomotive's total weight is supported without contributing to propulsion—potentially leading to wheel slip or reduced pulling power under load.^[32] Furthermore, the rearward concentration of this extra weight heightens axle loading, which has historically strained lighter tracks, limiting the suitability of such designs for networks with axle load restrictions.^[33] With the transition to diesel and electric locomotives in the post-1940s period, trailing wheels became obsolete as modern designs shifted to fully powered bogie arrangements where all axles contribute to traction via electric motors, eliminating the need for separate support structures like those required for steam fireboxes. This evolution favored simpler, fixed-wheel truck systems that offer better reliability and lower maintenance without the specialized pivoting mechanisms of trailing assemblies.^[34]

References

[1]
North American Steam Locomotive Wheel Arrangements
The last is the number of trailing wheels (typically under the firebox). For example a "2-8-4" or oOOOOoo means that there are two leading wheels (one axle), ...
[2]
Why Wheel Arrangements Matter On Steam Locomotives
Trailing wheels are located behind the driving wheels and support the cab and firebox. They can also provide additional traction when equipped with a booster ...
[3]
[PDF] Spring Equalization for Steam Locomotives - Doug A. Kerr
Oct 16, 2011 · trailing wheels. Such locomotives are often used as switchers;1 as ... Since very early in steam locomotive development, in most.
[4]
4-6-2 "Pacific" Locomotives: Development, Survivors, Photos
Nov 6, 2024 · Finally, the trailing axle provided for a larger firebox, which meant more steam pressure and ultimately more power as well. The Pacifics were ...
[5]
2-4-2 "Columbian" Locomotives: History, Photos, Survivors
Feb 18, 2025 · The trailing wheels' ability to support a larger and deeper firebox allowed engineers to use better combustion techniques, increasing the power ...
[6]
Tender Locomotives 0-4-2 “Olomana” - loco-info.com
The first known locomotives with this wheel arrangement date from 1834 and were built by Robert Stephenson & Co. for hauling freight trains on the Stanhope and ...Missing: history | Show results with:history
[7]
https://www.loco-info.com/view.aspx?id=-492
[8]
Articulating Some Articulated Facts - Train Collectors Association
... rigid wheelbase becomes too long, the locomotive cannot get around curves. In short, by 1851 locomotive builders were running into size and weight limits ...
[9]
R.0415 Curving - The Contact Patch
There was frequent flange contact, and derailments were common, so the question arose how to reduce the severity of the impact between the wheel and the rail, ...
[10]
Steam locomotive profile: 4-4-2 Atlantic | Classic Trains Magazine
Nov 3, 2020 · On the first Atlantics, the trailing truck axle was held by a rigid pedestal attached to the locomotive frame. It was soon replaced with ...Missing: American | Show results with:American
[11]
4-6-2 "Pacific" Steam Locomotives in the USA
It had piston valves, Walschaert valve gear and a one piece, cast steel frame. Number 5699 was built by the Baldwin Locomotive Works and was delivered in March, ...
[12]
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[PDF] machinery's 77717? - reference series w - Survivor Library
Intermediate driver and trailing truck equaliZer spring ..................................... 17,275 pounds. Trailing truck helical spring ... of the equalizing ...
[14]
Union Pacific Steam Locomotive Wheel Arrangements - UtahRails.net
Apr 10, 2025 · Steam locomotives on Union Pacific, like most of America's railroads, used the Whyte classification system for their wheel arrangement.
[15]
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Aug 1, 2025 · Every one of the 275 examples came equipped with boosters. The NYC led the way in this design, owning nearly 56% of all Hudsons ever made. Many ...
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Apr 12, 2025 · As late-era designs the J-3's were equipped with several new technologies including trailing truck boosters (increasing tractive effort) ...
[17]
Motive Power: Keeping Trains Moving - American-Rails.com
Nov 2, 2024 · Booster engine: Small steam engine on a locomotive's trailing truck or one of its tender trucks to provide extra power when starting a train.
[18]
Gliders, Yellow Jackets, and Stripes - NASHVILLE STEAM
Use of a four-wheel Delta trailing truck increased grate area and improved combustion and combustion chamber design, and incidentally dictated a 4-8-4 wheel ...
[19]
Berkshires and Kanawhas - Steam Locomotive dot Com
The Alcos had a conventional Delta trailing truck, but the Baldwins and Limas used the same sort of articulated trailing truck as A1A (There were ...
[20]
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Feb 27, 2025 · The articulated design was of the 2-6-6-6 wheel arrangement and based purely on horsepower and tractive effort nothing could top it. The ...
[21]
2-6-6-6 "Allegheny" Steam Locomotives in the USA
The large fire box was placed behind the drivers and required a six-wheel trailing truck to support it. This gave the design a wheel arrangement of 2-6-6-6.Missing: multi- | Show results with:multi-
[22]
Steel Castings, Then and Now - Railway Age
Jul 12, 2022 · Steel castings (side frames, bolsters, and couplers) are found everywhere on freight railcars and locomotives, and their supply is critical to the ongoing ...<|control11|><|separator|>
[23]
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... wheel arrangement (two pilot wheels, 8 drive wheels, and 2 trailing wheels to support a larger firebox). Engines built with this wheel arrangement were all ...
[24]
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Nov 5, 2024 · The American's Whtye Notation is broken down as follows: "4" lead wheels (two axles), "4" drivers (two axles), and "0" trailing wheels.At A Glance · Definitions And Terms · Steam Engines, Another Name
[25]
[PDF] Kentucky Jefferson L & N Steam Locomotive #152 Same 1837 East ...
L & N steam locomotive #152 ... (the large powered wheels), and two small trailing wheels at the rear which help support the weight of the firebox and cab.
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Wheel Notation | PRC Rail Consulting Ltd
This page describes the various systems used from time to time to describe the way in which the wheels are distributed under a locomotive.
[27]
Nickel Plate Road no. 765 - Fort Wayne Railroad Historical Society
Named for the mountainous terrain in which it was proven, the 2-8-4 Berkshire-type locomotive, with two pony wheels, eight driving wheels, and four trailing ...
[28]
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In addition to the added stability, the addition of the extra trailing truck allowed the locomotive to carry a larger firebox than the 2-10-0. While the new ...<|separator|>
[29]
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The booster represents an investment of about one-sixth of that of a locomotive, but it increases its tractive effort by approximately one-fifth. Consequently ...
[30]
Steam Locomotive Boosters
Learn how steam locomotive boosters, small auxiliary engines attached to trailing trucks or tender axles, enhance tractive effort at low speeds during starts.
[31]
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[32]
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Beginning with defining key terms, we will present equations relevant to the rail wheel dynamics of steam locomotives, take a closer look at the techniques and ...<|control11|><|separator|>
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The John Bull, in about 1833, was one of the first locomotives in America to be fitted with such a device to clear obstacles off the track. Soon, pilots became ...<|control11|><|separator|>
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Diesel Locomotives | PRC Rail Consulting Ltd
The modern diesel locomotive is a self contained version of the electric locomotive. Like the electric locomotive, it has electric drive, in the form of ...