3rd rail is where the train gets its electric feed from. If you see on the tracks, there are 2 rails where the train runs on and a "third rail" that is raised up and parallel that has an electric current running through. The train has a few contact shoes on each car that basically rub against it and gain electric current, allowing it to operate. This is for the red, blue, and orange lines. The green line uses catenary, which has direct contact from overhead wires and uses a pantograph system, rubbing under the wire to gain an electric current.