Subway trains get electric power from an electrified rail or line placed somewhere in the tunnel. Many trains get power from a third rail because they run inside short/small tunnels. Other trains get their electricity from an overhead power rail or power line because the tunnels are taller/larger.

The third rail is a rail that is usually built next to and higher than the two rails that support the train's wheels. The third rail will be on one side of the train but doesn't always stay on the same side of a train. Typically at stations, the third rail is farthest from the platform.

No matter if a train gets its power from a third rail or from an overhead wire, most subway train's get about 600 volt direct current electricity. Some subway lines have lower voltage power and others slightly higher. Because of this high voltage it is very dangerous. (For reference, car batteries are 12 volts, for example.)