What happens if the players prepare in advance is complicated.
How much advance warning do they have?
What stage of the plan are the dwarves at when the players start prepping?
How obvious is the players prep? Do the dwarves have inside men in the police or military ? Are they relying on what they can observe to warn them?
I have not done my detailed prep for this module as I am still on the train journey but if the PC's get information you need to sort out the above and then work out the terrorists reactions, I assume they continue the plan until they know it is blown then improvise as they are suicidal fanatics I think they continue the attack but if the subrail station is an obvious deathtrap they may decided to take their bomb and blow up somewhere else, pick somewhere were the showdown can be epic I like the Opera House , of if your city has one a children's hospital , maybe the local equivlant of Eton a really big school for the children of the elite. Basically anywhere spectacular with security unable to fight of the terrorists which probably means anywhere except the Governor's residence , RHC HQ, or military bases. Maybe combine the sneaking the bomb in with diversionary sniper attacks in other public places, or drop the bomb and hijack as many trains and ferries as possible and sink/crash them for maximum casualties historically ferry accidents are great for bodycounts.
Hopefully your players will be subtle and keep things quiet until they can locate the main terror cell and kill them otherwise I would make things messy.