Railway Electrification

Railway must operate at variable speeds using different electrification systems such as AC (alternating current) and DC (direct current).

DC is a constant flow of electricity which travels around a circuit in one direction.

Unlike a DC supply, an AC supply flows backwards and forwards: its direction continually alternates. The rate at which the current alternates, called the frequency, is measured in hertz (Hz). For example, in the UK, AC supply is 50 Hz: it alternates 50 times per second.

Electricity supply is generated at power stations, which use large generators. A generator converts mechanical energy into electrical energy. Current from the generators leaves the power station and enters the power grid – the network of power lines (cables) which transmits it around the country.

DC voltages between 600V and 800V are used by most tramways (streetcars), trolleybus networks and underground (subway) systems. The DC system is quiet simple but it requires thick cables and short distances between feeder stations.

3kV DC is used in Belgium, Italy, Spain, Poland, Czech Republic, Slovakia, Slovenia, Croatia, South Africa and former Soviet Union countries.

DC is also used in third-rail systems. Third rail is applied in small-diameter tunnels and subway systems. There is also a four-rail system, which is used in the London underground.

AC is only used on overhead wires, as it implies very high voltages. Inside the locomotive, the transformer steps the voltage down for use by traction motors.