A train is a series of connected carriages that run along a railway track. The carriages, also known as cars, transport passengers or cargo. A locomotive is the engine that provides the power for a train. It is the part that connects to the front or back of a train and pulls or pushes it along railway tracks.

While the terms are often used interchangeably in casual conversation, a locomotive is not the same as a train. A locomotive is a specific type of rail vehicle that provides the motive power for a train; it is essentially the "engine" that either pulls or pushes the unpowered cars. A train, by definition, is a connected series of vehicles—which typically includes one or more locomotives plus various carriages for passengers or wagons for freight. Think of the locomotive as the tractor and the train as the entire tractor-trailer combination. In 2026, many modern passenger systems use "multiple units" (like the London Underground or high-speed trains), where the motors are distributed under several cars rather than being concentrated in a single locomotive at the front. In these cases, there is no separate "locomotive" at all. However, in traditional rail setups, the locomotive is the heavy, powerful machine at the head of the line, while the "train" refers to the entire assembly of cars traveling together along the tracks.