A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use electric locomotives to haul passengers or freight in separate cars or electric multiple units, passenger cars with their own motors. Electricity is generated in large and relatively efficient generating stations, transmitted to the railway network. . Some electric railways have their own dedicated generating stations and transmission lines but most purchase power from an electric utility..

Power is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of two forms: overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings; third rail mounted at track level and contacted by a sliding "pickup shoe".

Disadvantages include high capital costs that may be uneconomic on lightly trafficked routes; lack of flexibility - since electric trains need electrified tracks (or overhead wires)- and a vulnerability to power interruptions. Different regions may use different supply voltages and frequencies, complicating through service and requiring greater complexity of locomotive power. The limited clearances available under overhead lines may preclude efficient double-stack container service.

In comparison to the principal alternative, the diesel engine, electric railways offer substantially better energy efficiency, lower emissions and lower operating costs. Electric locomotives are also usually quieter, more powerful, and more responsive and reliable. They have no emissions, an important advantage in tunnels and urban areas. Some systems provide regenerative braking that turns the train's kinetic energy back into electricity and returns it to the supply system to be used by other trains or the general utility grid. While diesel locomotives burn petroleum, electricity can be generated from diverse sources including renewable energy.

Railway Electrification System