Characteristics of Railway Electrification

Electric railways use electric locomotives to haul passengers or freight in separate cars or electrical multiple units, passenger cars with their own motors. Electricity is typically generated in large and efficient generating stations, transmitted to the railway network and distributed to the trains. Some electric railways have their own generating stations and transmission lines but most of them purchase power from general electric grid. The railway usually provides its own distribution lines, switches and transformers.

Power is supplied to moving trains with a conductor running along the track that usually takes one or two forms. The first is an overhead line or catenary wire suspended from poles or towers along the track or from structure of tunnel ceilings. Locomotives or multiple units pick up power from the contact wire with pantographs on their roofs that press a conductive strip against it with a spring or air pressure.

The other is a third rail mounted at track level and contacted by a sliding “pick-up shoe”. Both overhead wire and third-rail system usually use the running rails as the return conductor but some systems use a separate fourth rail for this purpose.

In comparison to the diesel engine, electric railways offer substantially better energy efficiency, lower emissions and lower operating costs. Electric locomotives are usually quieter, more powerful and more reliable than diesels. They have no local emissions, which is an important advantage in tunnels and urban areas. Some electric traction 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 grid. While diesel locomotives burn petroleum, electricity is generated from diverse sources including many that do not produce carbon dioxide such as nuclear power and renewable forms including hydroelectric, geothermal, wind and solar energy.

Disadvantages of electric traction include high capital costs that may be inefficient on lightly trafficked routes; a lack of flexibility since electric trains cannot operate on non-electrified tracks and a vulnerability to power interruptions. Different regions may use different supply voltages and frequencies which causes complications to train service. The limited clearances available under catenaries may prevent efficient double-stack container service. The lethal voltages on contact wires and third rails are a safety hazard to track workers, passengers and trespasses.