Probably this addition appears on other threads on this excellent forum. However, to the general question posed at the beginning, I must point out that a market existed for through Washington-Boston service more than a hundred years ago, and the railroad industry provided it.
You might look at this explanation of the Poughkeepsie Bridge route:
For a while, the Federal was floated across New York Harbor to Oak Point or some other landing spot in The Bronx.
While we rode between GCT and Winsted in the late 1940's, Dad would point to Pennsylvania Railroad equipment on some trains from Boston to Washington. I don't know whether the equipment pools were in full sets or half New Haven, half Pennsylvania, but this forum certainly has people who know about that.
In the late 1940's, Bridgeport-to-Penn Station was a higher fare than to GCT, but not as huge a difference as today.