The monorail explodes before I can get on it

Solution 1:

You killed the mole too fast.

If you enter the watchtower and wait for the mole to finish their check-in on the radio you will hear about their plans to destroy the monorail and gain the opportunity to disarm the monorail's explosives before it can detonate.

If you talk to Col. Hsu before disarming the bomb or kill the mole before learning of the bomb's existence you will be unable to disarm it before the monorail leaves and is destroyed.

You can read more about the layout of the quest from the Fallout Wikia page

Solution 2:

What you're supposed to do is, when you enter the tower, don't move. Stay crouched and don't let the spy hear you. He'll start talking and give away the plan to blow up the monorail.

Thankfully, New Vegas keeps a backup of your quicksave slot, which I was able to restore, go back, and figure out how to salvage the situation with console commands.

If you've already killed the mole and talked to Colonel Hsu, you can fix the situation by executing the following console command:

set "000e790f".bPlayerTooLate to 0

After executing that command, enter the monorail platform and you'll be able to disarm it, it won't leave.

Also, I encountered a glitch where when in the tower, the mole doesn't talk on the radio. He just stands there until you go up and kill him, which is probably why I didn't realize you were supposed to eavesdrop the first time. To fix this, I left the tower and executed:

set "000e790f".bCurtisAmbush to 0

When I entered the tower again I stayed crouched and didn't move, and he talked on the radio as he was supposed to. I was able to complete the rest of the quest normally.