A train is a connected series of vehicles for rail transport that move along a track to transport freight or passengers from one place to another. The track usually consists of two rails, but might also be a monorail or maglev guideway. Propulsion for the train is provided by a separate locomotive, or from individual motors in self-propelled multiple units. Most modern trains are powered by diesel locomotives or by electricity supplied by overhead wires or additional rails, although historically the steam locomotive was the dominant form of locomotive power. Other sources of power are possible. The word ‘train’ comes from the Old French trahiner, itself from the Latin trahere ‘pull, draw’.