Take out the bad guys from your elevated position before dropping in if you want to control the situation, then head to the van. After that, bafflingly (or inconveniently), you’ll face a brief section as Briggs, but in First Person mode. You can avoid the engineers by sticking to the sides of the tunnel and hiding behind the train cars, go through the one at the end of the tunnel and come at the van from that side to get the drop on everyone.
The first section of Sam isn’t too rough. Stick close to the side of the building where you start, and just leapfrog the train cars until you get to the other side. Then follow the wall, up the stairs, and you can take out the sniper, who is also an HVT! Watch out for mines, but otherwise it’s a snap.
From the sniper perch, climb up and over the wall to the next area. Look for a commando having a conversation across from you. You can climb the wall and shimmy over to him and drop on him. Move out of the covered area into the railyard, and hug the right wall. There will be one guy in your way, and a number of mines. Don’t mess with the mines, just climb over things to stay out of their way. At the far end of the railyard it is all shadowy, which is very reassuring. Play leapfrog with the trains again until you get near your marker. In the marked train is an armored guard and an engineer. Beyond them is the bomb.
Now you’re Briggs again. Look to your right, climb through the train car. On the other side is a guy. Take him out and stick to cover. When the sniper is looking away from where you are, stand up and RUN to the open door. You won’t have enough cover to sneak there. There are a lot of bad dudes in the roundhouse, it’s easier to pick a fight and have them come to you. If you don’t want to, hug the wall to your right and go up the stairs. There’s a sniper at the far end, and beneath him is your exit point. There’s a gun in armor wandering around in here, too.
Now you and Sam go for a train ride. It is combat heavy, and very straightforward, so I won’t ruin it for you.