An electric locomotive is a locomotive powered by electric motors and drawing power from an external source (through overhead lines or third rail), or from an on-board storage device such as a battery or a flywheel energy storage system. Locomotives with on-board prime movers, such as Diesel engines or gas turbines, are not classed as electric locomotives when they use electric motors to turn the wheels, as the electric generator/motor combination is only considered to be the power transmission system.