Head up the ladders to the surface, (in the truck there's explosives), head for the large double doors - hit the switch on your right and follow the corridor to the broken down car. Go to the gate on your right and break the padlock, open the Steam Tunnel Access hatch and head down.
Follow the corridor, opening the door by the valve, until you get to some steam vents, stay to the left of the steam and get to the lever in between them (this will turn the steam off here and further on). Continue down the corridor and through some doors, until you come to a lift. Take the lift up to the Warehouse Basement and follow the corridor until you come to a "T" junction, turn right and head around the corner. Time it carefully and you can get through the electricity bolts. Pull the metal crate across to the vent opening to jump up into it, crawl through the vents to the other side.
Head left for a stash of goodies, then return and go right. Listen to Harold the scientist who advises you to find Dr. Rosenburg, head through the door opposite the room. Follow the corridor to the Main Stairwell, head up to the First floor's Storage room. Go outside to the left, jump on the skip, then tires to get through the window. Go into the room on the left and head down the corridor. In the warehouse you'll hear a scientist shouting for help from inside the red container, open this up from the side furthest from the track doors. Head into the Storage Room then out to the Freight Yard.
Open up the red container with another scientist, by shooting the wooden wedges under the large coils of cable. Soldiers will come out from behind a yard door (there is a .50 cal machine gun mount in the green box opposite the door), head down the tunnel. Jump into the back of the truck to get the rocket launcher to destroy the tank (4 rockets, aim a the top). Go to the far side of the yard and into the Maintenance Access door.
Follow the corridor to a tunnel where there is a train carriage near a locked door. On the carriage is 4 gas canisters, shoot the tips to launch them and blow a hole in the door. Climb onto the carriage at the end furthest from the door. Go into the Turntable Controls room, open the door and turn the table, head over to the door you just opened and go down the right hand side, activate the train. Open the red carriage to find Dr. Roseburg.
Use Dr. Rosenburg to exit the carriage from the ceiling, and defeat the soldiers. Re-open the carriage and get Dr. Rosenburg and go to through Freight Warehouse and into the stairwell, up to third floor Shipping Receiving. Follow the corridor to Freight Records then along and down another stairwell to the Basement. Dr. Rosenburg will stop near a boarded up wall, which reveals a lift when destroyed. Take Dr. Rosenburg with you along the corridor to open the door, to meet Walter. Follow Dr. Rosenburg to the teleportation chamber next door, watch as he fires up the teleporter, then jump in when he tells you.