London Underground - A Loud Exit
Once you regain control, the guards will make their way back inside. For these first two, just take your time and save ammo. You need to be able to kill three more, so go for headshots. Once they're dead, the figurehead will fall and you'll get the map out of it. Once you regain control, get behind a pillar and open fire again. If you're out of ammo, you'll need to take them out in hand-to-hand. Use the cover of the railing and the pillars to get close, then sprint up the steps and start swinging.
At this point, Charlie will finally note that you're all low on ammo and having a silencer isn't much of an advantage. Pick up a .45 pistol from one of the dead guards, and get some ammo from the others. When you're ready, go out the door.
At the first open area, you'll be ambushed. Note that there are several laser-sighted shooters in back. You can't really target them, so just watch carefully and duck back down if they get the laser dot over you. You'll get off to a good start by following the railing to the left. This will get you away from the shooters in back, and let you flank the first group on the floor. Move up to a container to take out the rest.
Later fights will all be in the same style, so take your time now to get used to it. Loot guns and ammo off of the ground, and look for the assault rifle that one of them should be carrying. When you're ready, move up to the old train yard. Take cover on the first train to the left, and fire down the line with your assault rifle. You'll be fairly safe from the guards with laser-sights in back, but you will have to deal with rushers. If anyone tries to run past you, or actually makes it around, use blindfire to aim in their general direction. At this range, you'll kill them quickly.
Once you pick off the shooter on top of the other train, it will be fairly easy to patiently clear out the rest of the room. The laser-sighted shooters will fall back, and you'll have a moment of peace. If you want, stop here and look at the ledges beneath the clocks. One of them has a piece of treasure below it. If you shoot it, it should fall down and let you pick it up.
Advance through the station and follow the way up. Watch for a guard with a shotgun at the top of the steps. Be ready to fire quickly from the hip to kill him. I would actually recommend swapping out the assault rifle for the shotgun. It may only have two shots, but at close to medium range, it's an instant and easy kill.
Regardless of your choice, swing around and engage a few more guards. After a bit of a firefight, you'll eventually reach a group of laser sight shooters positioned behind a gate. You can't hit them and they're not going anywhere. Just fire quickly to kill the nearest guards, and then run out of cover and go up the steps. Shoot or punch the guards that you run into, then make it out to the next station. You should come out at the North Atwood station.
After killing the first guard you meet, walk down the left side of the train on the tracks with your weapon out. You should be able to aggressively make your way down the tracks, and flank the guards waiting behind the train. There's a piece of treasure lying on the tracks just at the end of the train, so grab it and go up the steps.
Move up and around. Kill the last batch, then watch your back as you make a run for the exit at the top. A few more guards will come from behind, spin around and kill them, then just make a run for it. You should be able to trigger the cutscene and end the mission.