The Elmer Bumpus bridge was built in 1982 - is it because there wasn't a good crossing over the tracks there historically?
This is the exact alignment I was thinking of when I started working on the map in 2021 (image also includes a Green Line branch to South Station and the Seaport). The portal would be south of the Mass Pike, not under Eliot Norton Park.
The traffic island on Tremont @ Marginal. They'd have redone the intersection so the buses portaled up in the middle of the road. The SL III tunnel was actually going to come down Charles St. South in the last plan before the plug got pulled. Underpinning the Tremont trolley tunnel was a cost blowout so that alignment got scrapped pretty early on.
It's easier tunneling than going straight down Shawmut because the new underground Tufts Station wouldn't be as constrained (don't forget, the old portal was 4 tracks so if you ever wanted to have multiple branches here you'd ideally want to build Tufts as a twin-island station) and you have a more favorable angle for underpinning the Orange Line. Plus that intersection of Tremont/Marginal sets up a future multi-directional junction if you relocated the E Line off Copley Jct. with a Back Bay Station + Marginal subway. You'd be able to use the quad-track, flying-junction Tremont tunnel to avoid the traffic conflicts of Copley Jct. and be able to thru-route Huntington service to South Station and the Seaport, which is big if you ever build out the Huntington Subway to Brookline Village as a Central Subway alt-spine. That junction features a lot in various Green Line Reconfiguration crayon maps.
The parcel between Tufts and the Doubletree was originally going to be the portal where buses would turn out onto Washington Street.
I'm not aware of any such studies that have been publicly released. I don't know how far the original light rail proposal got in planning, but it's my understanding that the Shawmut Ave subway never really got very far, with the SW Corridor being preferred from very early on in the planning process.
I think what I’m remembering, at least for the light rail option, is this: United States Department of Transportation/Urban Mass Transportation Administration. Replacement/Transit Improvement Study: Alternatives Analysis and Draft Environmental Impact Statement Evaluation, Draft June 1987
1972 Boston Transportation Planning Review. The complete study isn't available online, only the Executive Summary. Which is weird that it hasn't been scanned out of the State Transportation Library yet because it was a major big-deal comprehensive planning study that influenced immediate action on the SW Corridor redesign post- I-95 cancellation, the Red Line northwest extension, and the Big Dig.
