A locomotive, originally a locomotive engine, is a power unit used to haul railroad trains. The earliest development of the railroad locomotive took place in the United Kingdom, where Richard Trevithick built his first engine c.1804. Robert Stephenson family's famous Rocket of 1829 proved that locomotive engines were far superior to stationary ones and provided a design that was archetypical for the remainder of the steam era. Locomotives were first built in the United States c.1830. These pioneered many new design features including the leading truck, a set of wheels preceding the main driving wheels, guiding the locomotives over the usually lightly-constructed American tracks. For most of the rest of the nineteenth century, locomotives of the 'American' type (4-4-0) were standard on US passenger trains, though towards the end of the century, progressively larger types came to be built.
Although electric locomotives have been in service in the US since 1895, the high capital cost of converting tracks to electric transmission have prevented their widespread adoption. Since the 1950s, however, most US locomotives have been built with diesel engines. Usually the axles are driven by electric motors mounted on the trucks, the main diesel engine driving a generator which supplies power to the motors (diesel-electric transmission). Elsewhere in the world, particularly in Europe, much greater used is made of electric traction, the locomotive usually collecting power from overhead cables via a pantograph. Although some gas-turbine locomotives are in service in the US, this and other novel power sources do not seem to be making much headway at present.