MAP OF THE PRESIDENTIAL METRO - SOUTHERN PART:
Map legend: 1 - passageway to the northern section; 2 - holodisc; 3 - power box; 4 - metro cart
You can check the rest of the tunnel leading south if you want to. You would have to deal with a small amount of radiation, but you would find a few interesting items, including a power fist and pulse grenades. Proceed towards the passageway located on the eastern wall. Once inside get rid of the final group of ghouls [#46] and you'll soon receive confirmation that all hostiles have been eliminated. I would recommend going back to the Presidential Sub Level and informing M.A.R.Go.T. about your success. In exchange you'll find out that the train can be used again.
Return to Presidential Metro and continue moving south. Eventually you'll get to a large junction. Start off by exploring the western corridor, because you'll find a holodisc helpful in finishing a small subquest. In order to complete this quest you would have to travel to Georgetown - east after your return to Capitol Wasteland. You would then have to enter Le Maison Beauregard hotel, kill Lag-Bolt, take his key and open a nearby briefcase. This object would contain a unique outfit. :-) Once you've collected the holopad go back to the junction and this time choose the eastern corridor. Approach a nearby power box [#47] and wait for a friendly sentry bot to repair it for you by replacing a fuse. If the robots are hostiles towards your character you would have to destroy the bot, find a fuse on your own and install it in the power box. Enter the train and use the main lever to be transported to a new area.
You'll find yourself standing in an area called Adams AFB Metro Exit. You won't spend too much time here, however I wouldn't recommend moving too quickly. Choose the corridor leading west. Upon your arrival in a new area you can decide to open a nearby door with a bobbypin and hack the terminal to activate a security turret. You would also find a stash here, containing microfussion cells and pulse grenades among other interesting objects. You can also ignore the side corridor completely and attack Enclave soldiers [#48]. Killing them shouldn't be a problem. Once they're all gone proceed north and find the door leading outside (to Adams Air Force Base).
You've finally returned to the ground level. The base itself is quite large, however there is going to be only one main path leading to your current destination. I would recommend that you leave the main road from time to time in order to find new equipment and to hack into security terminals. Start off by using the stairs. There's a small building to your left [#49] and you should consider entering it. There are some supplies waiting for you inside, as well as the first terminal. I would strongly recommend that you start hacking into terminals, because this will allow you to shut down nearby (not all!) security turrets.
Exit the building and check the area behind it. It's EXTREMELY important that you find a ressuply crate [#50]. Aside from the ammunition and a short note with new orders you'll also acquire a Tesla cannon (!!!). This is by far one of the most powerful weapons in the game and obviously you should start using it right away. If you've been spending a lot of time exploring stashes and looting bodies you should have plenty of ammunition for your new toy. Use the cannon to attack enemies from a large distance, because in close quarters combat you would risk taking injuries yourself.
You can start off by destroying a couple of security turrets using a Tesla cannon [#51]. The turrets can be found on roofs of some of the nearby buildings. You can gain experience points easily that way and you won't have to worry about the turrets firing back. You should also know that in most cases a single hit will be more than enough to destroy the target.
Keep heading north, however watch out for enemies equipped with flamethrowers. You may encounter them very soon and despite what you might think their weapons are also going to be effective from larger distances. The main reason is that they're using upgraded versions of flamethrowers that fire some kind of fireballs. As a result you'll have to focus on eliminating them as soon as possible, especially if your armor doesn't offer good protection against fire. You should also know that you'll be entering a sector with new turrets. Travel north-east [#52]. Locate a large container and enter it to find a computer terminal. You can use the terminal to disable a new set of turrets.