The link to old MBTA maps on Wikimedia eventually leads you to the
1963 MTA Master Plan of Rapid Transit Extensions. Generally speaking, most of it is pretty unsurprising, mostly stuff we've talked about for years.
With one
bonkers exception...
View attachment 5887
As far as I can tell, that is a proposal for an extension south from Ashmont to a park-n-ride at Granite Ave. While it looks intriguing on paper, I'm guessing the idea was abandoned for environmental reasons -- the Neponset River is
right there, and I'm pretty sure there are wetlands in that area as well.
BERy scratched at that itch way longer ago when they were studying conversion of the Old Colony Shawmut Branch to Red Line extension from Andrew (built as Ashmont + High Speed Line for 1928).
At that time the Granite Ave. station site across the river on the '63 map was where the Old Colony's Milton Branch forked off the mainline at North Quincy, where it formed sort of a mirror-image inland commuter 'circuit' to the Shawmut Branch on opposite side of the river. Back in the 20's the OC mainline was still way too busy for consideration except for the 5+ track segment through Savin Hill shared by Shawmut Branch running alongside which the RR was willing to part with. So it was thought that the easiest way to hit Braintree with rapid transit was going straight out of Ashmont/Cedar Grove, over the Neponset on a new drawbridge, and taking over the Milton Branch to Quincy Adams before rejoining the mainline for Braintree. Plan didn't get very far because unlike the Shawmut Branch which was almost exclusively commuter the Milton Branch still had economically significant freight to the Quincy granite quarries (Milton Branch being the final configuration of what originally started in 1826 as the Granite Railway, North America's first-ever chartered RR). I don't think this one ever got far enough along for BERy to have a dedicated study done for it; the granite quarry freight was a hard blocker. Documentation of it as a future consideration is limited to couple passing footnotes in various annual reports tracking planning progress of the Ashmont extension.
Had that gone through you'd have no RL branches, no Quincy Ctr. touches, but a linear mainline that went Harvard-Ashmont then maybe a Cedar Grove trolley transfer or some other treatment of the Mattapan stub...then Granite, East Milton Sq., West Quincy, Quincy Adams, and North Braintree (split w/ Greenbush Line...Plymouth/Cape split is more commonly placemarked as "South Braintree").
Milton Branch ROW is still traceable today. Go to North Quincy Station, exit north headhouse. Enterprise Dr. is the ROW, and the driveway to the big parking lots is the old mainline junction. From end of Enterprise it follows the north property line tree berm of Harris Ave. and the Presidents Golf Course, crosses Granite Ave., and follows the south south property tree berm of the MassHighway maint yard. From there I-93 literally swallows the ROW as its roadbed. Service ended in 1954 so they could start tearing things up for the highway build, and some of the expressway overpasses near East Milton Sq. were the result of major rebuildings of the overpasses of the old (double-widened for highway) RR cut. ROW reappears independent of 93 just south of the Furnace Brook Pkwy. exit rotary, at the Sullivan Rd. dead-end. Traceable alongside Boston Financial Data's driveway, crossing Village Dr., splitting the Beth Israel parking lot, and side of the road on Congress St. and alongside BJ's. Tracks still extant from BJ's passing underneath Burgin Pkwy. to the mainline switch, where it's called the West Quincy Industrial Track. Until the mid-90's when most of this industrial area started being flipped to big boxes, this was active all the way to Furnace Brook and you could see cars stuffed at the end of the line through the trees while driving 93N. Whatever former industrial park LLC owned this last fragment simply forgot to file for abandonment of that rump of trackage after BJ's when the last customer at the site of the now-vacant Lowes was bulldozed in 2007-08 s to build the Lowes. Today it sits nominally active with connected mainline switch (albeit fenced off), and nobody has returned the T's phone calls in years about permission to remove the switch.