DC electrification supply system design

Railway electrification has in the past been dominated by overhead contact wire and DC third/ fourth conductor rail electrification systems. The historical reasons for this have been the success of the DC traction motor and the necessity of a DC supply. Mercury arc rectifiers were originally used to provide rectification at substations with the DC power being transmitted to the traction equipment by the conductor rail or overhead wire. Success in producing mercury arc rectifiers capable of being operated on board the railway vehicle, enabled railway AC electrification systems to become a reality in the 1950/60´s. It should be noted that DC is still the most common form of railway electrification system in the world.