Apropos of infrastructure required for multiple classes of service on a route, in 1984 the Japanese Keisei line operated 4 classes of service on 2 tracks Ueno (Tokyo) - Narita: no stops, 1 stop, a dozen stops, all stops. Riding the "dozen stops" train, we overtook 2 or 3 locals with cross-platform transfer in 4-track, 2 island platform stations. The midpoint had 7 (IIRC, could have been 8) tracks and 4 platforms. The no-stops train zipped through while the other three classes were doing cross platform transfer through the semi-express cars (doors open to platforms on both sides).
Yes, the investment and culture change to get there from where the MBTA is now would be considerable, but it is definitely possible.