Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th century to distinguish between mobile and stationary steam engines.

A locomotive has no payload capacity of its own, and its sole purpose is to move the train along the tracks. In contrast, some trains have self-propelled payload-carrying vehicles. These are not normally considered locomotives, and may be referred to as multiple units, motor coaches or railcars. The use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight (see CargoSprinter). Vehicles which provide 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 pull trains from the front. Increasingly common outside North America is push-pull operation, where one locomotive pulls the train from the front and another locomotive pushes it from behind. In this arrangement the locomotive at the rear of the train is controlled from a control cab at the front of the train. Push-pull operation is generally infeasible in North America as, even if mid-train or tail-end "helpers" are provided, the front-end might have over 26,000 horsepower, net for traction, whereas the mid-train and/or tail-end "helpers" might have only 9,000 horsepower, net for traction.