Train Puzzle：Electric description

An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device (such as a chemical battery or fuel cell). Electrically propelled locomotives with on-board fuelled prime movers, such as diesel engines or gas turbines, are classed as diesel-electric or gas turbine-electric locomotives because the electric generator/motor combination only serves as a power transmission system. Electricity is used to eliminate smoke and take advantage of the high efficiency of electric motors; however, the cost of railway electrification means that usually only heavily used lines can be electrified.