When the cablecar stops and the door opens, get out and get ready to fight the two guards.
You should be able to take out the one that stays in the office with a couple of headshots.
Use the available cover to take out the other guard as he attacks.
To get through the security door you'll need to enter the office by crouch-jumping
through the office window. Gather the items you find in the office, press the button
that opens the security door then go through into the corridor beyond.
Head up the corridor until you see the end door. You'll have to fight a number of grunts
in the warehouse beyond. Try to use the exploding barrels and cover from the crates to your
advantage and don't run into the open doorway on the left as the chamber beyond is guarded
by sentry turrets.
When you've dealt with the grunts in the warehouse take some time to gather the items
you'll find in some of the crates. Head out through the door on the right which takes you
into a small alcove with an air-duct intake.
Press the button on the far side of the intake to stop the fan then break the front
cover grille and crawl into the duct. Crawl along until you reach an opening on the left
with a ladder inside (don't touch the fan at the end of the duct) and climb up it. Crouch
to get out onto the catwalk.
You'll probably need to deal with the grunt in the guardroom to the right now. With him
out of the way you can use the lever on the desk to open the security doors in the chamber
below. You should also use the wall switch to activate and then de-activate the ceiling
turrets. Once they are de-activated they should remain that way so that you can safely
pass through the security chamber.
Head down the little flight of stairs and out into the chamber (be careful that the turrets
aren't still active) then go through the security doors and head up the corridor beyond.