It's not very often you get stuck where to go in an FPS (especially one as linear as this) but Doom3 has done it.
I'm on the Monorail level. I have left some guy in the train end exited, kill everything in the 1st small area and then gone into the adjacent area (which is a whole lot bigger) and got rid of and nastiness in there too. The only way to go (as Yazz once said) is up. This takes you into a little control-room. There is a locker with a keypad and another keypad which controls a door seal to keep the Mars atmoshphere out (I think that's what it does anyway). Also in this room is a window overlooking the area with the monorail in. I don't know the code for either of the keypads in this room and have search all 3 areas on this level for over an hour and still not found anything. I've also search Google for a while but none of the walkthroughs seem to be finished.
All help is appreciated