Unlike other metro tunnels where the trains have been rendered useless and the player character is forced to walk from station to station, or the train tunnel leading to the Pitt where a loading screen takes the place of an actual journey, the presidential metro line is usable.
About halfway down the second tunnel you can look through a fence and watch two feral ghouls get splattered by a metro train.
There is a massive battle between the feral ghouls and robots which the player character can learn about from M.A.R.Go.T. The numbers and strength of the ghouls can overwhelm the robots after awhile, so the player character should help out the robots or face getting rushed by the ghouls once they are done with the robots. Alternatively, if you are finding the ghouls too tough to make it through, they are fooled by the ghoul mask. This will give you the opportunity to take them by surprise or avoid them to save ammunition and stimpaks.
The fuses you need to activate the metro are on the presidential sentry bot. He will automatically replace the fuses once you kill all the ghouls (thus clearing the security breach). You may have to go back talk to M.A.R.Go.T again. She will tell you that the main repairs will now begin, and once you get back to the train, you will see the presidential sentry bot replace the fuse, enabling the metro.
The fuses can also be acquired by hitting the presidential sentry bot's combat inhibitor while it is facing a turret or sentry bot. The presidential sentry bot will attack the other robot and become hostile to you. You can then kill it and loot the fuses.
If you did not find the Senator's ID you can force the computer to reprogram with a high enough Science skill.
This area contains many Easter eggs, and one should take a second look at what appear to be dead ends.
At the junction where the working metro can take you to Adams AFB, there are two locked doors requiring a key to enter. There is no key to be found, however. The doors open when you kill the presidential metro bot, although this will of course make all other bots/turrets turn hostile. There is no loot behind these doors (just more bots). If you shoot the presidential metro bot in the combat inhibitor, causing it to frenzy, the other bots/turrets will attack it and not you. However, the doors will not open.
Past the M.A.R.Go.T. computer entrance, at the end of the tracks there's a gnome with a camera and on the tracks, are two teddy bears tied to the tracks with a note saying 'Help Us' between them.
If you have a high enough Science skill or you found the Senator's ID, M.A.R.Go.T. will tell you that the last time the Metro line was in use was 200 years and 8 months before the current date.
The cutscene in which you travel through the subway on the train isn't actually a train. The game equips you with a massive piece of headgear that resembles the inside of the train, with your player character running along the track at very high speed, simulating that you're moving through the subway on a train.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
pcps3xbox360 Occasionally, when the reaver is killed, it may fling back down the hall where it came from. [verified]
pcxbox360 Reavers in this area may be affected by a physics bug that causes them to jitter and shake rapidly. This makes them very difficult to target and rounds often pass through them without dealing any damage, even inside VATS. This may be related to the aforementioned bug, as when killed they often fly some distance. Explosives still have full effect. [verified]