Crawl through to the hallway that seems like it will never end. Run around the hall until you hear the double click, then run back and a new area will be accessible. Climb down the ladder and push the button to go through the hall to the other side of the fence; once there, kill the subhumans and the guard with the flamethrower. Make your way to the control room and push the button to turn on the power. Climb on top of the control room, hang from the edge, and shimmy to the ladder. Climb down the ladder until your feet are at the bottom, then backflip, spin and grab the ledge to find your first secret.
Back at the mine carts, take the one on the middle track first. When going around turns, pull the brake to slow yourself down, but not so much that you won't make the jump. On your way down the track, hit the track switch, kill the enemies, and duck the drills. Slide down the slope backwards and grab the edge, then drop down. Watch out for the subhuman, hang and drop to the ledge below then work the crevices. Shimmy from one side to the other before dropping to the next edge. You will notice a passage, but watch out for the subhuman. Enter the next area and dispose of the enemies. You will now notice the crawl space. To get to it, shimmy across the wall and then push the button to open the door. On the walkway above the water, retrieve the crowbar and make your way across the tops of the snow hills to the passageway in the wall. Climb the ladder and go through the drills. Duck your way under the rock crushers and up the ledges. Drop down to the tracks and jump back into the car and make your way back to the beginning.
Use the crowbar to open the door and retrieve the lead-acid battery. Now it's time for your next ride. Jump into the cart on the lowest track and be sure to have lots of speed. When the cart stops, make your way down the hallway and drop into the steamy room. Continue, but watch out for the ambush. Follow the dark hallways, go through the corridors to the crane and quickly swim down and pick up the wrench. Head back to the cart. Back at the beginning, take the cart on the upper track. Down the track you go, ducking under the beams and the drills. Where the cart stops, place the battery in the crane and use the wrench to lower the submersible pod. Jump and swim down into the pod, hopefully not suffering too much damage. Pick up the medi pack, head back into the water and go through the passage between the two green lights. Make a right turn and swim up the long tunnel to the surface.
Dispose of the flamethrower, drop down below the bridge, and watch out for the freaks. Jump back and forth from ledge to ledge to find a secret and backtrack up to the structure. When you enter, the level ends.