Traction power network

A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is only done if the railway in question uses alternating current (AC) with a frequency lower than that of the national grid, such as in Germany, Austria and Switzerland.

Alternatively, the three-phase alternating current of the power grid can be converted in substations by rotary transformers or static inverters into the voltage and type of current required by the trains. For railways which run on direct current (DC), this method is always used, as well as for railways which run on single-phase AC of decreased frequency, as in Mecklenburg-Western Pomerania, Saxony-Anhalt, Norway and Sweden. In these areas there are no traction current networks.

Separate power for traction apart from industrial power always has historic roots. There is no reason today to apply different frequencies or current types than for transmission and for industrial usage. However, the advantage with DC traction was the easier transmission with single copper wires to the feeder points. The advantage with AC traction is the easier transmission over long distances to the feeder points. Beyond these parameters and securing former investment, no evidence exists to stay with different current schemes in networks.

Dedicated traction current lines are used when railways are supplied with low-frequency alternating current (AC). The traction current supply line is connected to substations along the line of the railway and is usually run separately from the overhead catenary wire from which the locomotives are fed.

In countries in which the electric trains run with direct current or with single-phase alternating current with the frequency of the general power grid, the required conversion of the current is performed in the substations, so again no traction current lines are required.

...
Wikipedia