I'm joining this thread as a freeloader

Thing is: We (Mrs. Bruce and I) will be travelling around the east coast in mid september. We'll have two and a half days in DC, one day in Baltimore, and one day in Philadelphia. Oh, and we'll be travelling by train.

Now, the question about DC and Baltimore has already been asked, so let me add my own question:

Is there anything (which is easily reached by train) on the road between DC, Baltimore, Philadelphia and NYC that you'd consider to be a must-see? Mrs. Bruce desperatly wants to visit Priceton, but don't ask me why..