To what is the title referencing? To what is the title's significance?

The Five-Forty-Eight is a reference to the train Blake caught to take home. It reminds Blake of the bomb shelters in the not so distant past, and it metaphorically represents the "bomb" that will go off in the train..... Miss Dent's emotional outburst.

Blake drank a second Gibson and saw by the clock that he had missed the express. He would get the local— the five-forty-eight.

Blake took in settling himself in an uncomfortable seat on the five-forty-eight deep underground.