Some quick thoughts...
1) Back Bay station is more like a London Tube station ... than a long distance train station..... so I recommend starting at South Station.
2) Back Bay also has an MTA station in it... and I've used the Marriott across the street and taken the subway from there to the airport and have seen a doorman's chins drop when they offer to get me a cab and I say, I think I'll walk across the street etc. Works fine if you're traveling light (which the OP may not be doing).
3) Union Station in DC has a great food court in it (in the basement) and when I'm in DC I often go there for lunch or dinner.
Bob H
Last edited by BobH; May 29, 2012 at 12:50 pm