Objective 1: Gain Access To Lower Cave System
Once the lift lands be careful and move forward quietly so you won't be noticed by the guard that walks back and forth. Crouch and hide behind the rocks. You'll notice there are two guards on your way, one that is patrolling and the other is guarding the elevator to your left. You'll also see a camera w/c FYI cannot be destroyed. Do not worry for you won't be using that elevator, there's a metal door at the front of the truck ahead of you... that's where you're going. In order to get there though you have to shoot the guy near the elevator while the patrolling guy is going the other side or you can also shoot them both (quietly and fast). There are some guards far to your right and also inside the near building and they'll come if they hear some shouting or gun fires. After that go to the metal panel and open it, this will lead you inside the ventilation shafts all the way down to the mines.
Objective 2: Disable Security System
Your objective here is to go to the office on the left part of the cave where the security system must be deactivated. It's the elevated room on the left side. You'll see two guards moving back and forth just in front of you. I suggest not to shoot them because there are other guards who might notice some ugly bodies on the floor. Wait till they turn their backs then move slowly (crouch mode) to the stone outcrops (shadow of the stones to your right). While the 2 guards are still moving far from you go straight to the space between the crates and the small house/shack a little to your left. After that cross the tracks and the train silently and hide behind the crates.
There are 3 guards talking to each other here, but they won't notice you if you crawl up to the stairs. What you have to be careful with is the single guard that patrols near the office. Use your thermal goggles to see the guard's movement. When the area is clear quietly crawl/crouch to the office and disable the security system. Don't run coz there's a guy hiding on the other door.
Objective 3: Open Doors To Tunnel
Go back and still check the patrolling guard on your way down. Your next target is the guy guarding the switch that opens the tunnel doors. Move around the small shack near the tunnel doors and wait for the pair of patrolling guys on the other side to move away. Now silently sneak behind the guard and pop him once in the head with your SD. Turn the switch and run (that's right RUN) back to the train and activate it.
Objective 4: Ride The Train To The Ventilation Shaft
Once the train moves jump to the middle coaches of the train and duck or crawl because you will be joined by another train with 2 guys in its cabin. Wait till the train stops, jump and move back along the tracks till you see the ventilation shaft to your left.
Objective 5: Use Ventilation Shaft To Gain Access To The Weather Station
For this just open the shaft's access door.