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

A locomotive is a railway 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. 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. 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 their trains. Increasingly common these days in passenger service is push-pull operation, where the locomotives push the trains in one direction, and are controlled from a control cab at the opposite end of the train in the other.


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 the steam locomotive, but still has some relevance.
  • 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.
  • 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:

Rolling stock
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An illustration of a historical train. Pictures provided by by Classroom Clip Art (http://classroomclipart.com)
An illustration of a historical train. Pictures provided by by Classroom Clip Art (http://classroomclipart.com)

The first railway locomotives (19th century) were powered by steam, first by burning wood, later coal or oil. 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.

The first steam locomotive was built by Richard Trevithick, and first ran on 21 February 1804, although it took some years before steam locomotive design became efficient and economically practical. Fairy Queen, built in 1855; plying between New Delhi and Alwar in India, is the longest-running steam locomotive in regular service in the world, but John Bull, built in 1831, is currently the oldest operable steam locomotive. John Bull is preserved in mostly static display at the Smithsonian Institution in Washington, DC.

The all-time speed record for steam trains is held by an LNER Class A4 4-6-2 Pacific locomotive of the LNER in England, number 4468 Mallard, which pulling six carrriages (plus a dynamometer car) reached 126 mph (203 km/h) on a slight downhill gradient down Stoke Bank on 3 July 1938. Aerodynamic passenger locomotives from other countries such as Germany and the United States attained speeds very close to this, and this is generally believed to be close to the practicable upper limit for the direct-coupled steam locomotive.

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. British Rail figures showed the cost of crewing and fuelling a steam locomotive was some two and a half times that of diesel power, and the daily mileage achievable was far lower. 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 seen little use.

By the end of the 20th century, almost the only steam power still in regular use in North America and Western European countries was on railroads specifically aimed at tourists and/or railroad enthusiasts, known as railfans 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 into the late 1970s. Steam locomotives are in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. India has switched in the last decade 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 most simple 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.

It has, however, 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.


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. In effect, such a locomotive is an electric locomotive which carries its own generating station along with it.

Early diesel-electrics were switching engines used to move rail cars around in rail yards. The first went into service in 1924. A decade later, the technology first began to be applied to regular rail service as streamliners went into service. Actually, a gasoline-electric system powered the first such train, but diesel-electric systems soon proved to be more cost-effective because of higher efficiency and lower maintenance costs. The fuel for one early high-speed run from Chicago, Illinois to Denver, Colorado only cost US$14.64 (in 1934]] dollars).

In the 1970s, British Rail in the United Kingdom developed a high-speed diesel-electric train called the High Speed Train or HST. This train consists of two Class 43 locomotives (also known as power cars), one at each end, and a number of Mark 3 carriages (usually 8). A complete HST set was originally designated as a Class 253 or 254 diesel multiple unit (DMU), but due to the frequent exchanges between sets the power cars were reclassified as locomotives and given class number 43. The unpowered carriages were simultaneously reclassifed as individual coaches - the number of a DMU set should identify all its associated carriages as well.

The prototype HST (designated Class 252) holds the world speed record for diesel traction, having reached a speed of 143 mph, although the operating speed of the production HST in service is 125 mph (200 km/h), hence the name "Inter-City 125".

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


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. Many diesel-hydraulic multiple units also have a "fluid flywheel" which acts as a "second gear" for running at higher speeds.

The inner rotating part of a torque converter is called a centrifugal pump (or impeller), the outer part is called a turbine wheel (or driven wheel), and between them is a fixed guide wheel. All of these parts have specially shaped blades to control the flow of oil.

The centrifugal pump is connected directly to the diesel engine, and the turbine wheel is connected to an axle, which drives the wheels.

As the diesel engine rotates the centrifugal pump, oil is forced outwards at high pressure. The oil is forced through the blades of the fixed guide wheel and then through the blades of the turbine wheel, which causes it to rotate and thus turn the axle and the wheels. The oil is then pumped around the circuit again and again.

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 reliabily 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 only diesel-electric locomotives of the Deutsche Bundesbahn were BR 288 V 188, of which 12 were built in 1939 by the Deutsche Reichsbahn Gesellschaft.

Gas turbine

Locomotives powered by gas turbines were developed in many countries in the decades after World War II. These used jet-type engines (similar to the turboshaft engines in a turbine helicopter) driving an output shaft. The normal method of transmitting power to the wheels involved an electrical transmission similar to a diesel-electric locomotive - the turbines running at constant speed driving a generator, feeding to large electric motors driving the wheels.

Gas turbine locomotives are very powerful, but also very noisy (they sounded rather like a jet aircraft). Union Pacific operated the largest fleet of turbine locomotives and used them extensively, at one point claiming that the turbines hauled 10% of the railroad's freight. Their efficiency was quite low, but this was initially not a problem; fuel was cheap, and Union Pacific's gas turbines were fuelled with cheap 'Bunker C' heavy oil. This cheap fuel source vanished when improved refinery techniques allowed it to be 'cracked' into lighter petroleum grades. After the oil crisis in the 1970s and the rise in fuel costs, gas turbine locomotives became uneconomic to run, and many were taken out of service. This type of locomotive is now rare. None are in use in the United States.


Main article: Electric locomotive

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 diesel ones, and are capable of superior acceleration as well as regenerative braking, making them ideal for passenger service in densely populated areas. Almost all high speed train systems (e.g. ICE, TGV, bullet train) 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 7MWatts 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 (750V and 1500V); Netherlands (1500 V); Belgium, Italy, Poland (3000 V), and the cites of Mumbai and Chicago (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 shutning 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


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

The newest technology in trains is magnetic levitation (maglev). 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 has reached 552 km/h.

The transrapid maglev train connects Shanghai's airport with the city.

The first commercial maglev trains ran in the 1980s in Birmingham, United Kingdom, 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. There are passenger locomotives, freight locomotives and switcher (or shunter) locomotives. These categories mainly depend on manoeuvrability, traction power and speed. Some locomotives are designed to work in mountain railways.

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