That is different than my version: An early detonation is less destructive, perhaps only collapsing the subrail tunnel, but it certainly kills any PCs nearby. The danger gets progressively worse, and by the time the train reaches Golden Point the bomb is primed enough that a detonation would create a thirty-foot wide crater. If the party manages to successfully cut all seven wires to phlogis-ton, they can keep the bomb from getting any more dangerous, but it will eventually go off unless they can deactivate the whole thing. The choice is up to the party.