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.
 * Jumping down directly will cause damage. However, you can try jumping onto one of the metal carts.
 * 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.

Survival
The region within survival covers the train building, just before you reach the emergency door. Before opening the door, you have access only to the first room.

The best defensive location is within the second half of the map. Collect shotguns and pipe bombs, group in front of the door, and charge as one unit. Head up the stairs, and all the way to the left. Back up onto the far wall, with two members watching the stairs and the other two watching the top. Tanks will normally come by the ground, but can come by the top as well.