What if we extend the Red Line to Milton, and then run Type 9 trains from Milton to Mattapan (and maybe continue the Type 9s along the 29 bus route (minus its Franklin Field Housing deviation) to Jackson Sq if sufficient transit reservations can be set aside in the streets)?
Alewife effectively uses one of its tracks to turn around Braintree trains and the other to turn around Ashmont trains, which suggests that a single track at Milton might be sufficient to turn around all trains, and if turning around every train that currently serves Ashmont on a single train at Milton isn't practical, sending only half the Ashmont trains to Milton and having the other half terminate at Codman Yard during rush hour would also be possible.
Single tracking a Milton terminus would simplify the interlocking there to a single switch, assuming double track was preserved from the Neponset River to Ashmont and beyond.
If we do this, I think we should also consider building a pedestrian underpass in the vicinity of Cedar Grove, and side platforms between Cedar Grove and Gallivan Blvd to serve both current Cedar Grove riders as well as to give Gallivan Blvd bus riders an opportunity to transfer to the Red Line; a road diet of Gallivan Blvd at the same time may be desirable as well.
There isn't nearly enough facilities space to extend the line to street-running. The line runs on an 8-car fleet. You can't maintain something much larger than that at Mattapan shed or some LRV-serving replacement shed in the corner of Codman Yard. So unfortunately 28X is pretty firmly in the BRT category as best available option, because even though Blue Hill Ave. once
had a trolley reservation that shared yards with the HSL...it also had an Arborway Carhouse connection on the other end that kept the fleet requirements well-distributed. And that doesn't exist today, nor is there space for it.
N-O! on single-tracking anything. No rapid transit operation in Boston runs on low-enough margins for its frequencies--including the existing HSL--to survive with only 1 track. Braintree and Ashmont can and do appear on either
Alewife track when conditions warrant, even when most of the time they stick to the same side for wayfinding sanity. It's not foolproof; ops needs chaos-absorbing flexibility at turnbacks at any given moment.
Extending the Red Line to Mattapan in-full is not a big production. The two required grade crossing eliminations are not big productions. The state simply doesn't want to spend a dime more than it's forced to kicking-and-screaming on the corridor, and they have tried to bury it enough times prior that the residents have so much pent-up mistrust that public support is guided in large part by what prevents
another chance at transit loss. Support would probably be higher for the HRT option if they had any indication the state was dealing with them in any plausible honesty over it...but the state pretty much spit all over that option. If accepting Type 9's is what keeps the dreaded bustitution talk tabled for another 20 years, that's where the widest share of support gravitates. It's not because they think that's 'best'...they know they can do better, and that includes fact that the trolleys could run a whole lot more frequent than the CAF's propping up the same not-very-frequent headways as the PCC's. But it is 'safest' pick when they haven't gone more than 10 years without dodging a bullet on major existential threat of transit loss.
Hopefully the next debate after this punt the conversation will actually pivot to what's a greater transit deal for the corridor, but right now those residents can be forgiven for still treating it like survival mode.