It used to be every other Acela (weekdays) or thereabouts that stopped at BWI. Very useful and I used it several times from BOS. The slight time savings (and more pleasant cars) made the trip from BOS just into the realm of possibility versus a Regional, and BWI is much easier for almost all of Anne Arundel County.

(Of course, once B6 and WN started going head to head on BOS-BWI it was hard to pass up the airfares most of the time, at least back in the Before Times.)