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A locomotive is a vehicle that provides the motive power for a railway train. Traditionally, the locomotive or locomotives are positioned at the front of a train, pulling passenger carriages and/or freight[?] vehicles. This requires the locomotive to be moved from one end of the train to another when a change of direction is required. However, it is now common for the locomotive for a passenger train to remain at the same end of the train. A driving cab is provided in the outermost carriage, with controls which communicate with the locomotive through wiring along the train. The train is thus pulled by the locomotive when moving in one direction, and pushed in the other. A variation of this occurs when a train consists of a set of carriages with a locomotive at each end, both of which are controlled by a driver in the leading locomotive.

Long freight trains sometimes have locomotives at the front and rear, and even in the middle of the train. This reduces the force on the couplings between the freight vehicles. In this case, control signals are usually sent from the leading locomotive by radio.

Trains which do not have locomotives are referred to as multiple units.

The first railway locomotives (19th century) were powered by steam engines. Because of this, the some people took to informally calling the steam locomotives themselves "steam engines". The steam locomotive remained by far the most common type of locomotive until after World War I.

Before the middle of the 20th century, electric and diesel locomotives began replacing steam locomotives. By the end of the 1960s, most countries had completely replaced steam locomotives in commercial service. Other designs, such as locomotives powered by gas turbines, have been experimented with, but seen little use.

Well before the end of the 20th century, almost the only steam power still in regular use in the USA and Western European countries was on railroads specifically aimed at tourists and/or railfans[?]. Steam locomotives remained in commercial use in parts of Mexico in to the late 1970s. Steam locomotives remain in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. In some mountainous and high altitude rail lines, steam remains in use because it is less affected by reduced air pressure than is diesel.

Diesel locomotives differ in the form of transmission used to convey the power from a diesel engine (or engines) to the wheels. It has been found impossible to design a mechanical gearbox which is suitable for the power produced by most diesel locomotive engines. 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; 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.

Alternatively, diesel-hydraulic locomotives use hydraulic transmission to convey the power from the diesel engine to the wheels. Few locomotives of this type are in use nowadays, though it is used commonly for multiple units.

The all-time speed record of steam trains is held by an A4 Pacific locomotive pulling six cars (plus a dynamometer car) at 125 mph on July 3, 1938.

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