We tried to do this a couple of months ago. There was no sign of the bus, so we took a cab to the Amtrak station. The cab driver agreed to take us, but was extremely rude during the drive and accused us of lying about the train schedule. We arrived at the station five minutes before the train was scheduled to leave, only to find out that it would be an hour and a half late. We could take the Acela in half an hour, but it would cost $30-something per person instead of $9 for the regular train.

We went outside and hopped in a cab (a different one) who charged us $50 to take us into DC.