The quickest way to get this is the safe in the West Office. After you've used the combat knife to cut the tape on the box to the west wing of the station and raised the gate, head through the corridors and into the Operations room. Move through this room to climb up through the broken window.
Once you drop down, turn to the right and move forward, the door to the West Office will be in front of you. Go straight across from the door into the West Office to a side room which has the safe.
Combination is Left 9, Right 15, Left 7