In rail transport, a train is a series of connected vehicles that run along a railway track and transport people or freight. The word train comes from the Old French trahiner, derived from the Latin trahere meaning to pull, to draw. Trains are typically pulled or pushed by locomotives (often known simply as engines), though some are self-propelled, such as multiple units. Passengers and cargo are carried in railroad cars, also known as wagons. Trains are designed to a certain gauge, or distance between rails. Most trains operate on steel tracks with steel wheels, the low friction of which makes them more efficient than other forms of transport.