Moker's Shipping (map name: steel4)

Right at the start is a door marked "Keep Out". Go inside, load up, and head right back out (as if anyone could resist a door with such a sign). You'll need the ammo. Stuck after you find out Moker is not in his office? Look for some barrels on the catwalk above the side exit from his office. You can shoot them to get them out of the way.

Derailed (map name: ty1) Overview: Leave the train and watch your back. Go get the spare valve handle (the wheel thing)

and take it to the valve in the sewer room. Put it on, crank it open (activate it), and go on to the next map. The door at the end of this sewer (marked East D51) is locked, so activate the switch and climb up the ladder you'll see on your right about half-way down the sewer. The room you climb up into is where the broken valve handle is. There is also a working valve handle here, but turning it just closes the door you came through. To get past the train wreck in the tunnel, walk up the tunnel to the first two connected train cars. Look between them and you'll see another wheel shaped switch.

Activate it, and the second car will roll down and clear a path through the rubble for you. Walk past the rubble, watching out for the thugs hiding in the alcoves and doorways (there's tons of supplies in one of those rooms, so go load up if you need it). Once you release the car and clear the tunnel, go down as far as you can into the tunnel. Then you can duck underneath a boxcar, go left, and then jump into the boxcar to get some armor and bullets. Keep walking down the newly cleared train tunnel and enter a door on your left to get the spare valve handle. Take this back to the little room above the sewer and just walk up to the broken valve handle.

It will be installed automatically, and open the previously locked "East D51" sewer door below you. This leads to the next level.