I was figuring that extending it to Readville might be a bit too duplicative of future Dorchester Branch services, but the idea of possibly extending it to Dedham from there is intriguing. Somehow I don't think I ever considered RLX Dedham before, only OLX Dedham. Roslindale to Mattapan and Ashmont somewhat satisfies a circumferential market, and also beats out buses for people who need to get to one line or the other without going downtown. Perhaps BRT elements or streetcar reservations would be best suited to this circumferential routing.
Cummins Hwy. seems ripe for dragging DCR kicking and screaming into a lane-drop, since it's only 4 lanes from Mattapan Sq. to Mt. Hope Cemetery with the entire west half to Rozzie Sq. being a wide 2-lane + bike lane + parking row boulevard. For consistency's sake they might as well just do up the whole thing in that setup since the 4-lane section serves no discernible load-bearing function before its cemetery lane drop. Stripe a bus+bike lane across the complete corridor and signal-prioritize (not that it has all that many lights to begin with) and I think you've got a zippy bus route on-the-cheap. If local vs. radial stop-spacing needs to be differentiated you could even badge a CT
x route along there, since it might have that kind of demand profile if/when it ends up spanning a proper-Red HRT stop at Mattapan and a proper-Orange HRT stop at Rozzie.
Morton St. seems like the other that's screaming for circumferential BRT/CT
x-like service between a proper-Red HRT stop at Milton, Morton on an RUR'd Fairmount Line, and Forest Hills. Or maybe even stretching further east to a Braintree Branch infill @ Neponset/Port Norfolk pinging to Milton pinging to Morton pinging to FH if the dip down Adams St. doesn't chew too much time on a limited-stop roster. It would almost form an "Outer Circumferential Urban Ring" arc that way. But while I think you can safely sunset the 4-lane portion of Morton St. between Lower Mills and Gallivan Blvd. for similar bus+bike lane re-striping, Gallivan to Arborway being State Route 203 and a pretty consequential/non- induced-demand car artery means transit optimization is going to be a lot harder on the rest of the corridor. Maybe you could drag the neighborhood kicking and screaming into giving up some parking spots near intersections to stripe some protected left-turn lanes for flow, but 4 travel lanes are kind of necessary across that corridor.
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RE: "RLX" to Hyde Park, one of the cost savers for further extension to Hyde Park is the fact that the approx. 3000 ft. of subway under River St. would have zero stations in it while slicing through hard-ass Blue Hills base bedrock down a fairly wide street of sub-3 story homes set well back from the sidewalk via sizeable front yards. It would be TBM-able at complete uniform dimensions, and short enough to require at most only 1 emergency exit shaft somewhere in the Rector Rd. vicinity. When stations, utility relocations, waterproofing and/or soil stabilization, building/traffic/surface mitigations, any mid-bore dimensional changes, and the calendar itself for duration of any of those construction mitigations are the usual-suspect cost bloaters that end up dinging up any subway construction...this one sidesteps all of those risks. The interface with Mattapan Station and the infill plunked behind Price Rite and Match Community Charter School at the Poydras St. portal would be cosmetically similar to the Shawmut-Ashmont tunnel...both in length and in the way each portal is framed by structurally detached stations. So nothing the least bit radical by local construction standards. It's additional tunnel feet to contemplate for sure, but it's "cheap" tunneling methods-wise and we like that here when there are so few local corridors left capable of supporting any within-cost burial whatsoever.
And of course Fairmount Line all points south of River St. is contiguous ex- quad-track, with Fairmount Station + Fairmount Ave. overpass and the Dana Ave. overpass already having reserved berths and only the first two Neponset River crossings needing their more recent-construction footings to be buffed out. Then some type of flyover/flyunder gets you across about 800 ft. of Readville Station expanse onto the other side of Sprague St. and the Dedham Branch ROW. A flyover which you may be building anyway just with a Phase I extension from Mattapan to Readville if the storage yard siting for replacing to-be-retired Codman Yard @ Ashmont happens to pick Readville Yard 5 on the Sprague side instead of tucking next to commuter rail in Yard 2 on the shores of the Neponset. Rest to Dedham Ctr. is all academic save for approx. 1000 ft. at the high school where you're either relocating an athletic field driveway and installing an overpass crossing of a track cut or dipping very slightly sub-surface with tunnel roof supporting the driveway on its current layout (probably the better choice since this is a wide swath where school-age kids need to cross on-foot in large numbers, and pinching them onto a single overpass would be inconvenient and less safe).