Evacuation over
One of the plot devices was that they would not let the bomber know they found the bomb, so they could scare him into admitting something. So why would they let all the people back into town? Could they make noise, or go over to take a look, and ruin the trick? When they first were talking about diverting the train, they made it look like the siding was in a remote area. Then later it seemed that it was in the middle of thousands of people and buildings. I know it was done on purpose, but why was the church bell ringing so much? I have not diffused bombs, but I would seem to want complete quiet. The bells at midnight would have made me nervous - would it ring at 1 AM and 2 AM and so on also?
Why didn't the train crew want to listen to the station master? Gee, do I really have to park the train over there and unhitch the engine? Who are you to tell me that? Why didn't the police have more people at the Portsmouth station to catch the bomber? He almost ran away. Couldn't they have several cops at the far ends in case he ran for it? Would the bomber maybe go to the town before Portsmouth to avoid any traps? After all, he did get into a fight with one of the policemen guarding the stations, and he did drop his bomb making kit.
Now I am just speculating -- why couldn't they separate the cars with the mines and spread them out so that if 1 blew up it would not set off the whole bunch? That assumes you have lots of open space which seemed to be one of the main plot devices - that in fact there was not much open space.