The driving force 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 locomotives. Although historically dominated by steam propulsion, the most common types of locomotives are diesel and electric, the latter being supplied by overhead wires or spare rails.Trains can also be run by horses, pulled by engines or water-powered cable or wire beaks, run downhill using gravity, or driven by pneumatic, gas turbines or electric batteries.The track usually consists of two running rails with a fixed...