Railway electric traction supply systems| AC electric traction| DC electric traction| Direct current

Electric railways are a non-polluting energy efficient mode of transport for freight as well as people. Traction power for electric trains can be Direct Current (DC), Alternating Current (AC) or even a composite system employing both AC as well as DC.
A DC electrification system has several advantages. It allows rapid acceleration and braking of DC electric motors, DC systems cost less compared to AC systems. There is less energy consumption and they take less space and have lower weight. Importantly, a DC system does not cause electrical interference with nearby communication lines. However, DC voltage decreases with increasing length and therefore necessitating Expensive substations at frequent intervals typically 2-4 km. The overhead wire or third rail must be relatively large and heavy.
AC Traction has several advantages which have relegated DC traction to the side-lines. The AC power can be easily stepped up or down. It allows easy controlling of AC motors and the interval between substations is larger being between 15-20 Km. Moreover, it can use lighter catenaries and low currents can be transmitted at high voltages. Reduced capital costs of electrification. Like DC, AC traction also has some drawbacks such as increased costs of maintenance of lines and need for increased clearances in tunnels due to the overhead wires.
Composite System (or multi-system) trains are used to provide continuous journeys along routes that are electrified using more than one system.

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