The main fuse is a quest item added in Broken Steel. On the quest Who Dares Wins, you find a broken fuse box that activates the train to the left. A robot named Presidential metro sentry bot is carrying it in his inventory and it is required to fix the train. Should the robot survive the ghoul attack during the quest, he will eventually replace it himself, a rather easy workaround of it falling through the ground (see Bugs below for details).
pc ps3 xbox360 After clearing the area of all ghouls, the (optional) quest of containing the security breach may not trigger as complete, so the sentry bot with the fuse does not complete repairs. To fix either reload a save or just kill the sentry bot and do the repairs yourself and then the quest will finish. [verified]