Left 4 Dead/The Tunnel

You've entered the train station. As mentioned in the safehouse, you need to head north to the military base.

Route

 * Exit safehouse, and turn right to the exit sign. Head downstairs.
 * On this floor, turn left and climb out to the catwalks.
 * A ramp downward is available on the far side of the room.
 * It's possible to jump down directly, but you take damage.
 * The exit in this room is in the far left corner (from where you entered the room.)
 * Opening this exit triggers the alarm, alerting the horde.
 * Head down this generator room, there's an exit at the far side of this room. You can try exiting via the windows, but I don't see an advantage aside from height.
 * Once you reach the tracks, follow them - climb over any obstacles you encounter. You can either take ground routes or go across the tops of trains.
 * After climbing the ladder of a blocking train car, you should see an opening in the wall to your left - that's the next safe house.