A locomotive (from lat. loco motivus) is a railway [1] vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to move the train along the tracks.


In contrast, many trains feature self-propelled payload-carrying vehicles; these are not normally considered locomotives, and may be referred to as multiple units or railcars; the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see however CargoSprinter [2]). Vehicles which provide the motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.


Traditionally, locomotives haul (pull) their trains. Increasingly common these days in local passenger service is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is therefore optionally controlled from a control cab at the opposite end of the train. This is especially true of "High Speed Rail lines [3]", such as the Japan’s [4] Shinkansen [5] and France’s [6] TGV [7] trains.

Great Western Railway No. 6833 Calcot Grange, a 4-6-0 Grange class steam locomotive, at Bristol Temple Meads station, Bristol, England

Benefits of locomotives[]

There are many reasons why the motive power for trains has been traditionally isolated in a locomotive, rather than in self-propelled vehicles. These include:

  • Ease of maintenance - it is easier to maintain one locomotive than many self-propelled cars.
  • Safety - it is often safer to locate the train's power systems away from passengers. This was particularly the case for steam locomotives, but still has some relevance for other power sources.
  • Easy replacement of motive power - should the locomotive break down, it is easy to replace it with a new one. Failure of the motive power unit does not require taking the whole train out of service.
  • Efficiency - idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed.
  • Flexibility - large locomotives can be substituted for small locomotives where the gradients of the route become steeper and more power is needed.
  • Obsolescence cycles - separating the motive power from the payload-hauling cars means that either can be replaced without affecting the other. At some times, locomotives have become obsolete when their cars are not, or vice versa.

Classification by motive power[]

Locomotives may generate mechanical work from fuel, or they may take power from an outside source. It is common to classify locomotives by their means of providing motive work - the common ones include:


The main moving parts of a steam locomotive with Walschaerts valve gear: 1 - Link, 2 - Eccentric crank, 3 - Radius rod, 4 - Lap/lead lever, 5 - Crosshead, 6 - Valve, 7 - Cylinder, 8 - Reach rod

The first railway locomotives (19th century) were powered by steam, first by burning wood, later coke and coal or petroleum. Because of the steam engine, some people took to calling the steam locomotives themselves "steam engines". The steam locomotive remained by far the most common type of locomotive until after World War II. The age of steam correlates highly to the coal era.

See also[]

External links[]

Books on steam locomotives[]

  • C. E. Wolff, Modern Locomotive Practice: A Treatise on the Design, Construction, and Working of Steam Locomotives (Manchester, England, 1903)
  • Henry Greenly, Model Locomotive (New York, 1905)
  • G. R. Henderson, Cost of Locomotive Operation (New York, 1906)
  • W. E. Dalby, Economical Working of Locomotives (London, 1906)
  • A. I. Taylor, Modern British Locomotives (New York, 1907)
  • E. L. Ahrens, The Development of British Locomotive Design (London, 1914)
  • E. L. Ahrens, Steam Engine Construction and Maintenance (London, 1921)
  • J. F. Gairns, Locomotive Compounding and Superheating (Philadelphia, 1907)
  • Angus Sinclair, Development of the Locomotive Engine (New York, 1907)
  • Vaughn Pendred, The Railway Locomotive, What it is and Why it is What it is (London, 1908)
  • Brosius and Koch, Die Schule des Lokomotivführers (thirteenth edition, three volumes, Wiesbaden, 1909-1914)
  • G. L. Fowler, Locomotive Breakdowns, Emergencies, and their Remedies (seventh edition, New York, 1911)
  • Fisher and Williams, Pocket Edition of Locomotive Engineering (Chicago, 1911)
  • T. A. Annis, Modern Locomotives (Adrian Michigan, 1912)
  • C. E. Allen, Modern Locomotive (Cambridge, England, 1912)
  • W. G. Knight, Practical Questions on Locomotive Operating (Boston, 1913)
  • G. R. Henderson, Recent Development of the Locomotive (Philadelphia, 1913)
  • Wright and Swift (editors) Locomotive Dictionary (third edition, Philadelphia, 1913)
  • Roberts and Smith, Practical Locomotive Operating (Philadelphia, 1913)
  • E. Prothero, Railways of the World (New York, 1914)
  • M. M. Kirkman, The Locomotive (Chicago, 1914)
  • C. L. Dickerson, The Locomotive and Things You Should Know About it (Clinton, Illinois, 1914)

Electric and diesel-electric locomotives(general)[]

Before the middle of the 20th century, electric and diesel-electric locomotives began replacing steam locomotives. Steam locomotives are less efficient than their more modern diesel and electric counterparts and require much greater manpower to operate and service. As labour costs rose, particularly after the second world war, non-steam technologies became much more cost-efficient. By the end of the 1960s-1970s, most western countries had completely replaced steam locomotives in commercial service. Freight locomotives generally were replaced later.

Other designs, such as locomotives powered by gas turbines, have been experimented with, but have seen little use.


By the end of the 20th century, almost the only steam power still in regular use in North America [8] and Western Europe [9] countries was on heritage railways [10] specifically aimed at tourists and/or railroad enthusiasts, known as railfan [11] or train spotters, although some narrow gauge lines in Germany which form part of the public transport system, running to all-year-round timetables retain steam for all or part of their motive power. Steam locomotives remained in commercial use in parts of Mexico [12] into the late 1970s. Steam locomotives are in regular use in China [13], where coal is a much more abundant resource than petroleum for diesel fuel. India has switched in the 1990's from steam-powered trains to electric- and diesel-powered trains. In some mountainous and high altitude rail lines, steam engines remain in use because they are less affected by reduced air pressure than diesel engines.


Diesel locomotives vary in the form of transmission used to convey the power from a diesel engine (or engines) to the wheels. The simplest form of transmission is by means of a gearbox, in the same way as on road vehicles. Diesel trains or locomotives that use this are called diesel-mechanical and began to appear (although limited in power) even before the first world war which saw a number of simplex diesel systems built for the war, a small number of which survive and are still operational today. In the 1960s, the locomotive manufacturer Krauss-Maffei

  • [14] built a variant of this type of locomotive which used hydraulic power transmission for the Southern Pacific Railroad in the U.S., but the units were ultimately removed from service due to maintenance and reliability problems.

It has been found impractical to build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without breaking, despite a number of attempts to do so. Therefore this type of transmission is only suitable for low-powered shunting locomotives, or lightweight multiple units or railcars.

For more powerful locomotives, other types of transmission have to be used.


EMD SD70 series diesel-electric locomotives of the Union Pacific refueling at Dunsmuir, California

The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. This is effectively an electric locomotive with its own generating station.

Early diesel-electrics were switching engines used to move rail cars around in rail yards.

In the 1970s, British Rail [15] in the United Kingdom developed a high-speed diesel-electric train called the High Speed Train [16] or HST.

The HST holds the world speed record for diesel traction, having reached a speed of 148 mph, although the operating speed in service is 125 mph (200 km/h), hence the name "Inter-City 125".

A variant of the Intercity 125, the XPT [17], is in service on New South Wales [18] railways in Australia [19], but with a lower top speed and different carriages.

Diesel-electric locomotives come in three kinds:

  • those that cannot operate in multiple unit.
  • those that can operate in multiple unit with locomotives of the same kind.
  • those that can operate in multiple unit with most other diesel electric locomotives, as well as with straight electric locomotives.

Multiple-unit operation is more than standardising the cables that plug together between engines. It also means making the controls and characteristics of the locomotives compatible.

Most American diesel-electric and straight-electric locomotives use the so-called "American Association of Railroads" standard for multiple-unit control.


Alternatively, diesel-hydraulic locomotives use hydraulic transmission to convey the power from the diesel engine to the wheels. On this type of locomotive, the power is transmitted to the wheels by means of a device called a torque converter. A torque converter consists of three main parts, two of which rotate, and one which is fixed. All three main parts are sealed in a housing filled with oil. For further details see this artcle.

However, the range of variability is not sufficient to match engine speed to load speed over the entire speed range of a locomotive, so some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.

Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range. A fluid coupling is similar to a torque converter but the ratio of input to output speed is fixed; loading the output shaft results not in torque multiplication and constant power throughput but in reduction of the input speed with consequent lower power throughput. (In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears.) The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple unit compared to a locomotive makes this an acceptable tradeoff for reduced mechanical complexity.

Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics, but were found in many countries to be mechanically more complicated and more likely to break down. In Germany, however, diesel-hydraulic systems achieved extremely high reliability in operation. Persistent argument continues over the relative reliability of hydraulic engines, with continuing questions over whether data was manipulated politically to favour local suppliers over German ones. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in some European countries. The most famous diesel-hydraulic locomotive is the German V200 which were built from 1953 in a total number of 136. The only diesel-electric locomotives of the Deutsche Bundesbahn [20] were BR 288 (V 188), of which 12 were built in 1939 by the DRG [21].

The high reliability of the German locomotives was paralleled by higher reliability of non-German locomotives built with German-made parts compared to that of the same designs built using parts made locally to German patterns under licence. Much of the unreliability experienced outside Germany was due to poor quality control in the local manufacture of engines and transmissions, and poor maintenance due to staff used to steam locomotives working on unfamiliar and much more complex designs in unsuitable conditions and failing to follow the unit-replacement maintenance methods which were part of the German success. It is notable that diesel-hydraulic multiple units, with the advantages of modern manufacturing techniques and improved maintenance procedures, are now extremely successful in widespread use, achieving excellent reliability.

Gas turbine-electric[]

UP 68, one of Union Pacific's 4500 hp 'veranda' turbines. From the Don Ross Collection

Main article: Gas turbine-electric locomotive

A gas turbine-electric locomotive, or GTEL, is a locomotive that uses a gas turbine to drive an electric generator or alternator. The electric current thus produced is used to power traction motors. This type of locomotive was first experimented with in 1920 but reached its peak in the 1950s to 1960s. The turbine (similar to a turboshaft engine) drives an output shaft, which drives the alternator via a system of gears. Aside from the unusual prime mover, a GTEL is very similar to a diesel-electric. In fact, the turbines built by GE used many of the same parts as their diesels.

A turbine offers some advantages over a piston engine. The number of moving parts is much smaller, and the power to weight ratio is much higher. A turbine of a given power output is also physically smaller than an equally powerful piston engine, allowing a locomotive to be very powerful without being inordinately large. However, a turbine's power output and efficiency both drop dramatically with rotational speed, unlike a piston engine, which has a comparatively flat power curve.

Gas turbine locomotives are very powerful, but also tend to be very loud. Union Pacific operated the largest fleet of such locomotives of any railroad in the world, and was the only railroad to use them for hauling freight. Most other GTELs have been built for small passenger trains, and only a few have seen any real success in that role. After the oil crisis in the 1970s and the subsequent rise in fuel costs, gas turbine locomotives became uneconomic to operate, and many were taken out of service. This type of locomotive is now rare.


Main article: Electric locomotive

Electric locomotive VL60pk (ВЛ60пк)

The electric locomotive is supplied externally with electric power, either through an overhead pickup or through a third-rail. While the cost of electrifying track is rather high, electric trains and locomotives are significantly cheaper to run than diesels. They feature superior acceleration and regenerative braking, making them ideal for passenger service in densely populated areas. Almost all high speed train systems (e.g. ICE, TGV, Shinkansen) use electric power because the power needed for such performance is not easily carried on board. For example, the most powerful electric locomotives that are used today on the Channel Tunnel freight services use 7 MW of power.

The world speed record for a wheeled train was set in 1990 by a French TGV which reached a speed of 515.3 km/h (320 mph).

While recently designed electrified railway systems invariably operate on alternating current, many existing direct current systems are still in use—e.g. in South Africa, Spain, and the United Kingdom (750 V and 1500 V); Netherlands (1500 V); Belgium, Italy, Poland (3000 V), and the cities of Mumbai and Chicago, Illinois (which will be switched to AC by 2025).

A small number of electric locomotives can also operate off battery power to enable short journeys or shunting to occur on non-electrified lines or yards. Pure battery locomotives also found usage in mines and other underground workings where diesel fumes or smoke are not safe and where external electricity supplies could not be used. Battery locomotives are also used on many underground railways for maintenance operations as they are required to operate in areas where the electricity supply has been temporarily disconnected.

See also: Railway electrification system


Main article: Electro-diesel locomotive

These are special locomotives that can either operate as an electric locomotive or a diesel locomotive. Dual-mode diesel-electric/third-rail locomotives are operated by the Long Island Rail Road and Metro-North Railroad between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified as the third-rail power system is extremely hazardous in a yard area.

Magnetic levitation[]

Transrapid maglev train on the test track at Emsland, Germany.

The newest technology in trains is magnetic levitation train (maglev) [22]. These electrically powered trains have a special open motor which floats the train above the rail without the need for wheels. This greatly reduces friction. Very few systems are in service and the cost is very high. The experimental Japanese magnetic levitation train JR-Maglev MLX01 [23] has reached 581 km/h (361 mi/h).

The transrapid [24] maglev train connects Shanghai Pudong International Airport [25] with the city.

The first commercial maglev trains ran in the 1980s in Birmingham [26], United Kingdom [27], providing a low-speed shuttle service between the airport and its railway station. Despite the huge interest and excitement in the technology it was abandoned and replaced by a cable-hauled guideway a few years later.

Classification by use[]

The three main categories of locomotives are often subdivided in their usage in rail transport operations [28]. There are passenger locomotives, freight locomotives and switcher [29] (or shunting) locomotives. These categories mainly depend on manoeuvrability, traction power and speed.

Some locomotives are designed to work in mountain railways [30].

See Also[]


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