I like it too, but you have a big gap in density between Needham and West Roxbury due to nature preserves; that should never get developed. I also don't really see there being much need for local service between Needham and West Roxbury (as opposed to Newton and Needham, which already have economic, geographical and street connections). Lastly, I think there are very different frequency needs along that looped corridor: high along South Huntington, medium-high in West Roxbury, medium–low in Needham and medium-high along the Riverside line.
Combine that with the closure of yet another rail route into Boston (the Dover/Millis line), I'm not feeling it. Sorry. :-/
Needham Jct.-Medfield Jct. is already dead-dead-dead. T got ripped off yet again by the Iron Horse trail scam. $1/99-year trail lease is already signed...Iron Horse Preservation will be showing up in a year or two to cart off the scrap metal and leave a lumpy piece-of-crap 'free' trail behind. The only way to reach the (kinda-sorta active) Medfield-Millis portion is an out-of-the-way detour from the Franklin Line. Shame, because Millis has always badly wanted its commuter rail back and the projected ridership was one of the highest of any extension. But the Dover NIMBY's were NIMBY's when the service was still running and are an utterly immovable object.
Lamer: Medfield Jct.-Framingham is a nice convenient alt route up to Framingham.
Honestly, with how incredibly difficult it would ever be to de-landbank with the impenetrable opposition in Dover, there's no upside to keeping this commuter rail. Orange Line it to West Rox, Green Line it to Needham Jct. As long as the Needham Line's consigned to perpetual 'tweener status on the commuter rail, there's nothing they can do to increase service density. It's the odd man out for sharing the NEC.
Only the potential of a multi-branch mainline would've justified its permanent place in the NEC traffic mix, and that's now been shot to hell with the 2 potential routings past Needham Jct. salted over. I'd say let it go, replace the two halves with rapid transit, and extend the trail across 128 and Cutler Park to West Rox (because I don't think a 128 Orange park-and-ride brings enough all-day ridership) so it's actually a useful connecting rec route instead of a shitty-surface NIMBY scalping.
This was something I never liked about GLX. There is so much potential for that corridor all the way up to 128 to be rapid transit-worthy. (If the Red Line could've been extended to Arlington way back when...) But running LRT the 10 miles up to 128? I can't see anyone going for that. Yeah, it works for the Riverside line, but I think that's the exception, and moreover, it was essentially grandfathered in.
So extending the Green Line to Medford Hillside (or even West Medford) seems to close the door on expanding RT further north. And I don't like that. (Never close a door if you can help it.)
Provisioning is part of the reason why the stations are all being designed as prepayment with island platforms instead of D-like stops with onboard fares and track-crossing side platforms. They can be converted. Probably without much in the way of platform work since they could undercut the trackbed at most of them to get full highs. All of them have long enough platforms to hold 6-car HRT trains, there are no tight curves or clearance issues, and the electrical/signal conduits are being dug for easy modification to support future Lowell Line electrification...meaning it wouldn't be hard to switch over to 3rd rail power. Or...if they want to do it half-and-half like Blue, the Green Line overhead is 100% identical to Blue's and would be able to run HRT cars unmodified if the source power draw was beefed up accordingly. If they did a rapid construction job they could probably shut the line for 18 months and get a complete conversion.
All it really takes is getting the HRT line on-trajectory and preserving a parallel Green Line as far as the Union Branch + carhouse split. That's easy as hell if you branch Orange or something out of the N-S Link off the Orange portal and cut across BET.
The only moderately tricky parts for getting out to Anderson are:
-- Medford has to be grade separated at 4-track width.
-- The Route 16 and Mystic stone arch bridges have to be widened, but are historic structures. They'd do this by doubling the width with an exact-lookalike arch surgically joined to the old bridges to seamlessly look like a single-span arch.
-- All CR stations south of Anderson have to flip to rapid transit because there's no side-by-side room at any of them, and definitely no side room to do rapid transit + CR + freight passing tracks so the CR platforms can be all-high.
-- Winchester Ctr. overpass can't be widened to 4-track because of the surrounding density. That was actually the last grade separation ever undertaken on an active line on the current commuter rail network; B&M did it in 1960 so the tracks no longer bisected the rotary straight down the middle, and since it was modern-era construction there isn't a lot of leeway for more width. They'd probably have to leave the RR overpass intact for the Lowell Line and do a very short subway + subway stop underneath for rapid transit.
-- Rapid transit has to switch sides of the ROW from westerly to easterly because the freight customers start appearing en masse on the westerly side north of the Cross St. overpass (probably means your Winchester Ctr. subway stop is where the flip happens).
Other than that, it's ready-made for it as a mostly 4-track width ROW that's fully grade-separated and very well-buffered from abutting property. With plenty of yard space to plunk a large maint facility at Anderson. The 1945 expansion plan had the build proceeding almost exactly like that to Winchester Ctr. (with the eventual overpass being built 4-track from Day 1), only it cannibalized the (now obliterated by encroachment) Woburn Branch from there to Woburn Ctr. instead of staying on the NH Main.
