So an actual answer. There are two rails that the train rides on, these are a few feet apart and you can see them if you look down at the tracks. These are just metal and would be safe to touch (if there isn't a train coming). On heavy rail subway lines (not the green line) the train gets power from an energized third rail that is slightly to the outside of these two riding rails. It's also slightly elevated off the ground so that it doesn't short circuit to the ground. That is how you can tell which one is the third rail.