Someone had talked about this before on a thread and the ROW is not wide enough and Amtrak relies on it for emergencies and the Franklin line will need it to keep service levels reasonable in the future because of capacity constraints on the NEC.
Yes. Fairmount is definitely not wide enough north of Blue Hill Ave. to widen it out for anything extra. Just trace on Google Maps how densely abutted it is by residential. As EGE also notes, Franklin Line is going to need to gradually transition more of its schedule over to Fairmount over time as NEC slots get harder to come by in a couple more decades of exploding Amtrak and Providence traffic. Foxboro CR is already proposed to run full-time via Fairmount. Franklin circa 2040 is probably likewise going to be a full-timer on Fairmount that never touches the NEC (note: plenty of capacity to juggle Franklin + branches with Indigo).
And for various long-term reasons, you can't salt over the last freight route into Port of Southie when the 50-year economic considerations for ship-to-rail transloading would harm the regional economy if sealed off. Worcester Line's been purged of freight with Beacon Park's closure and is no longer a high-capacity option. Freights aren't allowed in the SW Corridor tunnel on the NEC, doubly so post-electrification. And when the Old Colony was rebuilt the fixed span over the Neponset River became the most severe RR grade in Massachusetts (the old fire-destroyed drawbridge used to sit waaaaaay below the Red Line bridge), ill-equipped for hauling anything heavy in/out of town. So you need Fairmount on the 12:00am-5:00am shift, the staging area at underutilized Readville, and the Franklin Line clearance route for dispersing goods all around 128- and 495-land.
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South of Blue Hill down to Readville is formerly a heavily industrialized area, evidenced by the still-scuzzy rump end of Hyde Park Ave. down there. Fairmount used to have a thicket of freight sidings down there keeping it at almost constant 4-track width, and the much-shrunken train yards down there used to have much fatter # of tracks and stretch further north than they do now (reason why that last Neponset crossing remains an active 4-tracker). You can see all sorts of trace evidence of the former rail-connected industrial property by the dam, by the still-vacant properties, by the sheds of the auto shops along the ROW, by the curious below-grade driveway next to the Fairmount station inbound platform, by Neponset crossing #2's ridiculously over-wide abutments...all the way down.
That is a rapid transit augmentation candidate that wouldn't foul any of the existing Fairmount passenger or freight capacity. But the only source for rapid transit is Red out of Ashmont/Mattapan...and some undetermined type of tunnel, El, or river-hugging embankment construction that closes the 1500 ft. lateral gap between the Mattapan and Fairmount Lines which was never filled by the RR's (Mattapan was Old Colony, Fairmount was competing New York & New England) in the 19th century before all that residential filled in.
And it would be a Hyde Park-serving line only, because Dorchester is the constrained part of the Fairmount Line that can't flip modes. This wouldn't rate as a high-priority extension given all the ones that are much more urgent, but it is wholly viable. Consider that Orange out of Forest Hills down Tracks 3 & 4 of the NEC used to be a viable studied extension for bringing rapid transit to Hyde Park. That's no longer possible because NEC growth requires all 4 tracks to keep feeding the Amtrak and Providence/Stoughton beasts through mid-century. So Red via Mattapan ends up accomplishing the same without gobbling anyone else's capacity. You just can't be all things to all people and get rapid transit on the west flank of Dorchester from Newmarket to Morton St. when 2 Fairmount RR tracks have a lot of mouths to feed. That's always going to be an Indigo route triaged with the best headways an xMU has to offer, and pumped up east-west bus route frequency. Doubt anyone's going to complain about that as a permanent condition when it's several orders of magnitude better than today.