A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as multiple units, motor coaches, railcars or power cars; the use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight (see CargoSprinter).
Traditionally, locomotives pulled trains from the front. However, push-pull operation has become common, where the train may have a locomotive (or locomotives) at the front, at the rear, or at each end.
The word locomotive originates from the Latin loco – "from a place", ablative of locus "place", and the Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, which was first used in 1814 to distinguish between mobile and stationary steam engines.
Prior to locomotives, the motive force for railways had been generated by various lower-technology methods such as human power, horse power, gravity or stationary engines that drove cable systems. Few such systems are still in existence today.
Locomotives may generate their power from fuel (wood, coal, petroleum or natural gas), or they may take power from an outside source of electricity. It is common to classify locomotives by their source of energy. The common ones include:
A steam locomotive is a railway locomotive whose primary power source is a steam engine. The most common form of steam locomotive also contains a boiler to generate the steam used by the engine. The water in the boiler is heated by burning combustible material – usually coal, wood, or oil – to produce steam. The steam moves reciprocating pistons which are connected to the locomotive's main wheels, known as the "drivers". Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in wagons called "tenders" pulled behind.
The first full-scale working railway steam locomotive was built by Richard Trevithick in 1802. It was constructed for the Coalbrookdale ironworks in Shropshire in the United Kingdom though no record of it working there has survived. On 21 February 1804, the first recorded steam-hauled railway journey took place as another of Trevithick's locomotives hauled a train from the Pen-y-darren ironworks, in Merthyr Tydfil, to Abercynon in South Wales. Accompanied by Andrew Vivian, it ran with mixed success. The design incorporated a number of important innovations that included using high-pressure steam which reduced the weight of the engine and increased its efficiency.