This should look familiar, since it's where you start the game. Climb up the train and jump off before it slides over the mountain, just like before. Afterwards, the action will pick up from where you picked up the dagger in a cut scene in the first chapter. The place is crawling with soldiers, so you'll have a hard time stealthing them all unless you're willing to run around a bit and be careful. If at any time you alert the bad guys, just head to higher ground. There's rail car next to the elevated train on the left side of the are, and it is an ideal spot, as you'll have high ground and cover from the direction in which they approach.
You start out right behind a guy, so take him down for an easy stealth kill, then head around the box car on the right, as you're facing where the soldiers are coming from. Don't go all the way around the car, but be looking around the side with the camera until you see a guard climb up the side to get an elevated position. Go up the same ladder he took, then shimmy over below him and use your stealth grab. Drop back down and continue to skirt the main crash site on the right. You should run into a guy patrolling over here. Wait until his back is to you, then take him out as well. One guy is standing out in the open, with his back to where you start, another guy patrols all along the middle of the area and two more patrol the opposite end of the area below the big building. Head back the way you came, and circle around to the guys on the other side. Get behind one of them as he patrols, then take him down as well. Hang out behind cover as far away from the middle of the area as you can and wait for the next guy to take out. If you climb up on the rail car roof over here, you can get access to the M32 - Hammer.
You can take out the roving patrol in the middle when he approaches, then go nab the last guy standing out in the open. After that, a new wave of enemies comes from over the wreckage and the hunt begins again. Remember to fall back to the far side of the area and take cover on top of the rail car by the train in the air if the bullets start flying. When you've dispatched those guys, a final wave comes over, this time the black armored guys. If you haven't used the Hammer yet, now would be a good time to break it out and lay some heavy fire on them. You'll know you've killed the last of them when Drake starts limping and walking slowly again. In the far corner of the area, behind the elevated train, you'll find the, and on the other far corner of the area, by the edge of the cliff, you'll find the . The last treasure is on the far side of the elevated train, away from the middle of the area. From down below you can shoot it down and get the . When you're done treasure-collecting, head to the flaming wreckage where the enemies came from and climb up the wall on the far left.