Underground trains can't burn fuel because the fumes would poison people in a confined space. Instead they must use electric motors, but the problem is how to get a continuous electric current to a train. Above ground, overhead cables are used because they are hard to reach and so less of a danger. Underground, they use a live "third rail". You can see it as the yellow strip below the train in these pictures. I watched as the train came in and every carriage had a contact at each end that rubbed along the third rail. I looked it up - apparently it is 750V DC. Notice that it is on the side furthest from the platform for safety.
