Updating to rails 2 2 2
Third (and fourth) rail systems almost always use voltages below 1 k V for safety reasons while overhead wires usually use higher voltages for efficiency.("Low" voltage is relative; even 600 V can be instantly lethal when touched.) Since utilities supply high-voltage AC, DC railways use converter stations to produce relatively low-voltage DC (usually 3000 volts or less).Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network and distributed to the trains.Some electric railways have their own dedicated generating stations and transmission lines but most purchase power from an electric utility.The railway usually provides its own distribution lines, switches and transformers.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".Electric locomotives are also usually quieter, more powerful, and more responsive and reliable than diesels.
Constant power with AC requires three-phase transmission with at least two ungrounded wires.Originally they used rotary converters, a few of which are even still in operation, but most were supplanted first by mercury arc rectifiers and then by semiconductor rectifiers.Because electrical power is equal to voltage times current, the relatively low voltages in existing DC systems imply relatively high currents.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 utility grid.While diesel locomotives burn petroleum, electricity can be generated from diverse sources including renewable energy.