Well, at one time, there was one Metroliner a week through from Washington to Boston.
I believe it was around 1991-1995 or so, but in addition to the New York NY to Washington DC Metroliners, there was one round trip each weekday from New Haven CT to Washington DC. It operated southbound in the morning, and northbound in the evening. On Friday evenings only, the northbound ran through to Boston.
To add some more trivia, Metroliner 201 use to start out of Downingtown PA and travel to Washington DC. It would deadhead in the wee hours of the morning from Philadelphia to Thorn Interlocking, change ends, and operate revenue from Downingtown to Washington via Philadelphia. This was before my time, but I heard stories about it from fellow Harrisburg Line Block Operators. I would think it was when the Metroliners were MU equipment since there is no where in the Downingtown / Thorndale are to “loop” or “wye” a train. I do not know if there was an evening return trip or not.