Actually one could almost go from Boston to Washington via commuter trains. We would have to take a short bit of Amtrak in Rhode Island between Providence and New Haven, but other than that it's commuter all the way.
Leaving Boston South Station. we could hop the MBTA commuter train to Providence.
Take an short Amtrak ride to New London then hop on a Metro North Shoreline East Commuter train to New York City, then hop on a NJT then SEPTA to Baltimore to pick up a MARC to Washington.
I don't know how long the trip would be, but it sounds like fun.
John
Can you get to Baltimore on Septa? I thought it only went to Wilmington.