Close to 50 years ago they *briefly* studied adding a rail median to Route 128 when the highway was being widened to 6 lanes all-around, but it was just a throw-shit-at-the-wall concept that wasn't even fleshed enough to say what mode they'd be choosing and what exactly (other than the generally-accepted rapid transit offsets for planned highway construction) it would interface with. Skin-deep only glance, so it left no mark or lessons on history.
The major problem with attempting this is that the density along 128 isn't a fit for transit. The immediate exits have all the clustered office park density, but between exits the highway is going through Neponset Reservation, Wilson Mountain Reservation, Cutler Park, outer Charles River, Stony Brook Reservoir, the outskirts of Minuteman State Park, and a lot of tall trap rock outcrops....all of which have more resident turkeys than humans. It's very hard to string something coherent together when the density comes on/drops off a table instead of ebbing and flowing in a way that would be amenable to transit. There's no "corridor" per se, just a density cluster built up around an exit that peters out when it hits the nearest geological limit. And since there isn't any sort of contiguous demand corridor, just a lot of last-mile trips from an exit, I can't see how a transit line that has to live 100% on transfers and the density that's immediately adjacent to the transfer stops can survive when there's almost no variety of intermediate stops you could string together for any sort of corridor. So I don't think there's any there there for circumferential transit, even if the spoke lines from Boston built/unbuilt/potential/crazy forced a taming of the car culture at those office parks. Last-mile shuttles are what's needed to the jobs, but they live off the spokes.
The only *possible* exception, and I think the need is a weak one, is shivving tracks along 128 for about 1.7 miles connecting Riverside Jct. on the Worcester Line with the Fitchburg Line as a replacement for the Grand Junction. There is easily room for it through Stony Brook Reservoir by shifting the road a lane over to the westerly side and rebuilding the River St. overpass. Assuming something is baked into the mass rebuild of the Pike/128 interchange you'd have the path reserved through the ramp spaghetti. But, as has been described here before, you don't need a direct replacement for the Grand Junction or the NSRL to take it offline for Urban Ring conversion; south vs. north Commuter Rail equipment independence such that 1-2x daily Grand Junction T/Amtrak moves can shrink to 1-2x weekly over the Worcester-Ayer bypass will do it. There's no freight considerations anymore for routing away from Allston. And the Worcester-North Station study showed little demand outside of hours when Orange/Red were suffering under load, so fixing Orange/Red reliability defrays most of that MetroWest need. I also don't think you're managing a Green Line poke to Waltham out of Riverside on this connector because River Rd. is an extremely weak intermediate catchment making it an extremely off-scale distance for GL stop spacing averages.
So...feasibility is pretty decently established. The need, however?...not at all. That connector would be a proverbial hammer in search of a nail.
