I am just about to step aboard the monorail when it zooms out of the station... boom! Could I have saved the monorail or was the bomb always going to detonate?

I'm wondering the same thing...

you have to find out about the bomb from eaves dropping on the spy, if your told about it by the NCR officer its too late.

The thing with this is that the quest is bugged, half the time the spy never finishes his dialogue and you have no idea what you're supposed to be doing. By the time I figured out what was going on from the officer and had walked up to the platform the monorail had taken off and been destroyed.

the quest is also bugged. fallout new vegas nexus has a mod that fixes it.

what i did is i wait until (i forgot the guys name;;) he went in to the tower. i followed him (make sure you enter crouched) litsen what he have to say(after hearing keep check in from the radio the quest mark should be updated) and dont kill the that guy ( since he have to keep talking to radio) and i stoled the passcode and deactivate the bomb..

this way i actually deativate the bomb.. (this was third walkthrough) didnt have any problem this time..

and i didnt download the mod to fix it.. so ..

Guess that explains it. Even though I was in the tower and listening, I couldn't stop myself from getting too close and then he quit talking to shoot at me.

When I did this quest, I kept talking to Hsu first and couldn't get there in time. When I used Turbo, I got there in time and was hitting the door to the train but it wouldn't let me in. It was still probably too late though because it was ready to go and I wouldn't have found the bomb and disarmed in time. I figured it was another bug and just let the quest go. Thanks for the other help on this thread for next time.

He finishes the dialog 100% of the time unless he notices you, try entering the tower while hidden.

Here's how I do the quest and so far I have never had it bug out.

Talk to Colonel Hsu and recieve the quest 'I put a spell on you'
Talk to the spy (ONLY this once)
Talk to the female NCR sergeant (The one from the interogation room)
Go straight to the Air Traffic Control Tower
Wait in the [Hidden] position until 1:00 AM-1:30 AM
Sneak in and eavesdrop on the spys radio conversation
Sneak out of the tower and run straight onto the monorail
Disarm the bomb
Report to Colonel Hsu

An important point here is to go straight from eavesdropping in the tower to finding the bomb on the monorail. If you stop to talk to Colonel Hsu it will fail every time.

Also, as mentioned, just go inside the door and stay at the foot of the stairs and wait while the conversation unfurls. You shouldn't have to go upstairs.

My monorail blew up and I guess I don't care because I wasn't in it. All these whiney NCR types walking around complaining about no way to get to NV. Ha Ha

Seriously, has any of you made it into the train but failed with the bomb and blew up? That would be so cool imo.