A train is a set of connected vehicles that travel along a railroad track to transport passengers and goods to their respective destinations. The track is comprised of two running rails with a fixed spacing in between. A self-propelled multiple unit with individual motors provides the motive power for a train. Historically dominated by steam propulsion, locomotives today are powered by diesel or electric fuels. Freight cars (wagons) are used in the transportation of cargo; a mixed consist is used to carry both passengers and freight in the same train. Passenger trains can travel both long and short distances. Examples include inter-city trains, regional trains, high-speed rails, commuter trains, trams, and many more.