Pretty self-explanatory. I just killed the guy that was radioing the Legion from the NCR base outside of New Vegas, and was wondering if there is anyway to save the monorail, or is it scripted to blow up?

Yes, you have to get to it and disarm the bomb before it goes off.

OK, how do I do that? Every time I get there it takes off before I can get on the train. Do I need to just run faster? I tried going on it before I went to Col. Hsu, but it just said to press "A" to go to New Vegas, so I assumed that wouldn't work.

IIRC, there's an air vent on one of the walls of the monorail car that the bomb is behind. If you mouse over it, you should get an "Activate" message that gives you the option to disarm the bomb.

This happened to me to. I think its a glitch. I believe I just reloaded to a different save. (Before I killed Curtis) Also if you reload the save, make sure you don't talk to Hsu

My mistake was that I killed him before I finished listening to his conversation with the legion on the radio. Luckily I hadn't deleted my save from before then. Is it better to kill Curtis myself or just disarm the bomb and turn him into Hsu?

The only way to find out who the spy is to go up and take a look, which means an automatic battle. I don't think there's a peaceful solution to uncovering his identity. At any rate, I always kill him myself and it doesn't seem to have any effect on the quest ending.

Cool. Can I wear the NCR armour I looted off of his body around the NCR, or will they attack me for being a fraud? I think I've got my reputation with them up to "liked" now after I finished this quest. Or, at some point will I get a chance to officially join the NCR and be able to wear their gear then?