There is your mistake you should have gone directly to the bomb. Its scripted to go off if you waste-time by talking to Hsu first. Its not a glitch but intentional but by pure luck you made it to the bomb in time when you weren't supposed to.
Hmm, this happened to me as well. It actually is a bug, because regardless of whether or not you are supposed to go to Hsu first, it allows you to get to the train, find the bomb, diffuse it and then gives you the message that it was successfully diffused.
Then the monorail blows up anyway . . .