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A train is a series of connected vehicles that run along a railway track and transport people or freight. Trains are typically pulled or pushed by locomotives, though some are self-propelled, such as multiple units or railcars. Passengers and cargo are carried in railroad cars, also known as wagons or carriages. 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. Many countries use rail transport.