I don't know if its been posted yet but there is a glitch with the new patch that will not let you on the monorail to disarm the bomb, the monorail just takes off right after you enter the room. I have used turbos to see if I could get on and take care of the bomb, but I am prevented from entering the train.

I'm betting its not a glitch but you simply failed the quest by either talking to Hsu the base commander instead of going directly to the monorall or telling Curtis you were going to investigate the break-ins before going to the tower.

it just won't let me in the train any way I do the quest. I deleted the game data and reinstalled the data without the patch and I could get in the train.

The latest patch was only supposed to add trophies for the upcoming DLC and restore the proper level cap. But on the off chance you might be right, I tested the quest. On my current character I hadn't done it yet. So I went ahead and entered Camp McCarren. I spoke to Hsu and received the quest. I then moved on to talk to Lt. Boyd where she told me about the break-ins. So I snuck into the tower around midnight where I overhear Curtis talking about the bomb. I then kill him lot his body and go directly to the monorall, I don't stop to talk with ANYONE on the way. I enter the train and disarm the bomb.
So I can only conclude the monorail exploded because you failed the quest somehow.

If you tell Curtis your going to investigate the break-ins. You Fail
If you don't overhear Curtis mention the bomb You Fail.
If you stop to tell Hsu about the bomb You Fail.