An electric train or an electric locomotive is a locomotive powered by electricity from an external source. Sources include overhead lines, third rail, or an on-board electricity storage device such as a battery or flywheel system. Electric trains in big cities are called EMU, electric multiple unit trains. The term multiple unit or MU is used to describe a self-propelling train unit capable of coupling with other units of the same or similar type and still being controlled from one cab. Image: Electric Multiple Units waiting to enter service in Poland.