A train is a form of rail transport consisting of a series of connected vehicles that generally run along a railroad (or railway) track to transport passengers or cargo (also known as 'freight' or 'goods'). The word train comes from the Old French trahiner, derived from the Latin trahere meaning 'to pull, to draw'. Motive power for a train is provided by a separate locomotive or individual motors in a self-propelled multiple unit. The term 'engine' is often used as an alternative to locomotive. Although historically steam propulsion dominated, the most common types of locomotive are diesel and electric, the latter supplied by overhead wires or additional rails. Trains can also be hauled by horses, pulled by engine or water-driven cable or wire winch, run downhill using gravity, or powered by pneumatics, gas turbines or electric batteries. The track usually consists of two running rails with a fixed spacing, which may be supplemented by additional rails such as electric conducting rails ('third rail') and rack rails. Monorails and maglev guideways are also occasionally used. Passenger trains include passenger-carrying vehicles and can often be very long and fast. High-speed rail systems began expanding rapidly in the late 20th century, and this remains a major subject of further development. The term 'light rail' is sometimes used to refer to a modern tram system, but it may also mean an intermediate form between a tram and a train, similar to a heavy rail rapid transit system. Freight (goods) trains use freight cars (or wagons/trucks) to transport goods or materials (cargo).