True - but the OP has now revealed that they want to visit both Princeton and the Yale - so they will need to now allow 2 days to get from DC back to NYC (pending details on the timing of visits to the colleges).

Thanks for all this information...very helpful! We've decided to just take train between DC and NYC. Then take a day trip to Yale by train during the NYC leg of our trip. And maybe not visit Princeton altogether...

I'm sure I'll have more transportation questions :) We are used to driving every where in CA.