This is obviously too late for the OP, but it may be useful to others:
On Acela, there is room in the café car for a double bass to stand upright since the Acela café car has no overhead storage and compact, bar-stool-style seating. You'll have to board early if you want one of those bar stools, though, since there are only 7 available; otherwise you may have to stand for the trip (which is fine; the conductor will still scan your ticket).
You can also take regular Northeast Regional trains. Each car has a narrow but full-height compartment at one end intended for luggage. If you are lucky enough to find an empty one, a double bass might be able to fit. At the end of each car there is usually a set of chairs facing each other that can (uncomfortably) seat four people. If the train is not full, you can probably place the double bass in this type of seat, perhaps with enough space for you to sit there too. There is also often an open area, usually by the lavatory, that I think is intended for passengers in wheelchairs. I often see people store large pieces of luggage there. If you are able to snag one of those seats, you could potentially prop the double bass on an angle against the wall. There is also a similar open area in the cafe cars. You will of course have to give up that area if a handicapped person needs it, though.