I'm trying to sneak up on the spy in the tower at night to listen up on his conversation but he doesn't say anything. He just stand there for hours forcing me to kill him. Is there anything I can do at this point to save the monorail? Because killing him and getting the activation code seems to send the rail running off to it's doom before I can get to the bomb.

You have to wait outside, at close to midnight. Crouch/hide/stealthboy. Wait for him to go inside, then follow him in. You will hear him talking, at which point you just turn around and run to the monorail. Don't mess around outside, run straight onto the train, or it will go off (and go off!).

wait for him at about 1am dont let him see you or he'll talk to you and walk off (thers some crates you can stand near about 50 yards away with full veiw of the door), have your followers wait, stealthboy + il fantoma mag, he'll start his chat as you get about half way up the stairs, pickpocket him for the codes, go to the mono rail (ive not rushed and always made it) go inside the monorail and at the front left side is a vent panel, enter disarm codes

go back to Hsu

My problem is he doesn't start his conversation with the Legion while he's in the tower at all. He just stay there in the tower all quite. Does he have to start talking for the bomb to be planted or can I just pick pocket him during normal hours and disarm the bomb at my leisure?

using stealth-boy and pickpocket activation code from him then.