Thanks guv1976.
I presume that we could check our baggage on day of travel, enjoy the free stopover (if available) and then collect our baggage when we arrived in Washington the next day.
I'll look into the stopover option as I think it would great to stop off at Philly en route. We're arriving into NYC before we head to Boston so will not want to stopover again en route to Washington.
One last question: how much luggage can you take with you on the train (i.e. unchecked) and must it fit in the overhead bins?
Thanks again. Very helpful.