How the hell do I arm the bomb. Im doing this quest for the legion I plan on destroying the monorail, I follow the steps given to me by the quest log, I get the ingredients and bomb from the garbage can, then I go to the monorail follow the objective marker to the outside back of the train car and I get no option to arm the bomb. It wont even let me enter the train car at all. Any help? I couldnt assisnt HSU to catch the spy because my reputation is horrible for the NCR (and for good riddance) and I coudnt convince him because my speech wasnt high enough so I decided to just blow up the monorail and it wont let me arm it.

The vent to place the bomb is inside the passenger car, in the back if I recall. I've only done one legion playthrough but it's where I remember it to be. (And if one does the quest for NCR instead of Legion, one finds a bomb in that spot that was planted by someone else)

All you have to do is go into the train, and come back out.

I had some problems with that quest myself. The first time I played it on the NCR side and tried to disarm the bomb, I couldn't get on the train to find the bonb. (so the monorail always blew up no matter how many times I retried).
When doing the quest on the Legion side, snuck into the base and had to contact Captain (whoever, cant't remember the name of the Legion spy), but he was alway marked as a red blip on my compass and would fire on me as soon as he saw me (tried this multiple times, even killing everyone else on the base before ttying to contact him).

Try it wearing NCR armor.