From Railroad Depot

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").

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 or railcars. The use of these self-propelled vehicles is increasingly common for passenger trains, but very 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 in local passenger service is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is optionally controlled from a control cab at the opposite end of the train.