In the Presidential Metro, you find a broken fuse box that activates the train to the left. This fuse box is fixed with the PP-PWR 10A FL25A RET MTR fuse.
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).
- When picked up, the character immediately drops it, then it falls through the worldspace (technically the ground).
- After using the train another fuse can be added to the sentry bot's corpse. This tends to repeat itself every time you go to use the train due to a loop in the AI.
- 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.