DC traction Units
Direct current (DC) traction units use direct current drawn from either a conductor rail or an overhead line. AC voltage is converted into dc voltage by using rectifier.
AC traction unitsEdit
All alternating current (AC) Traction units draw alternating current from an overhead line.
Main article: Multi-system (rail)
Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way to accomplish this is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another and so the train arrives with one locomotive and then departs with another. The switching stations have very sophisticated components and they are very expensive. A less expensive switching station may have different electrification systems at both exits with no switchable wires. Instead the voltage on the wires changes across a small gap in them near the middle of the station. Electric locomotives coast into the station with their pantographs down and halt under a wire of the wrong voltage. A diesel shunter can then return the locomotive to the right side of the station. Both approaches are inconvenient and time-consuming, taking about ten minutes. Another way is to use multi-system locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (1.5 kV DC, 3 kV DC, 15 kV 16 2⁄3 Hz AC, 25 kV, 50 Hz AC). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train coasts for a short time.