Sorry, I don't mean to appear flippant. I'm new to this, and I'm genuinely curious, so proposing a strawman seemed like a good way to learn more. If there are, like, books or podcasts you folks to get a base level of understanding of all of this, I'm here for it.
To clarify, my earlier comments did not intend to suggest your proposal was trolling or in bad faith; if that was the impression, I sincerely apologize. The part about vibe was mostly a feeling of awkwardness regarding the proposal, which I initially couldn't pin down explicitly (though I did eventually).
Yeah, this is the crux of my confusion. Quasi-rapid service via Regional Rail is the prevailing proposed solution for those gaps you listed, and some of those gaps are at least as deserving of rapid service as Quincy and Braintree are. So, if the Braintree branch weren't already in place, would there be advocacy in 2024 to build it, or would the advocacy be focused on delivering quasi-rapid service on the Old Colony trunk?
This is a very, very interesting hypothetical question. I'm gonna give a bold answer:
No, I don't think the Braintree branch would have been built today, nor even being considered nearly as seriously, had we missed the chance to do it in the 1970s. The primary reason would be that, in such a universe,
the 2 commuter rail tracks would have come first.
A lot of historical background of the Braintree branch is needed to understand the context here, which even I myself only learned from this forum. F-Line summarized it
here:
When the South Shore Branch was originally being planned in the mid-60's, they planned to make it separate from the existing Red Line and terminate on the surface near South Station (which, recall, was to be demolished and reimagined). The original intent was to replace commuter rail, not act as an appendage to rapid transit...as they were operating on the assumption that most of the NYNH&H Railroad would eventually be abandoned as the company went down in flames. This would allow extension to Weymouth, to Brockton, to anywhere they wanted to go. Basically, they were thinking of more of a BART-type second system than an organic extension of the Cambridge-Dorchester subway...merely cribbing the same rolling stock for a different RR-replacement purpose. And for that reason the downtown terminus was favored rather than running thru and branching Red Line frequencies.
It was a hotly debated decision, however, as there were lots of planners who did want it to interface with Red. So they hedged on a design for Columbia Jct. that would work with both schemes, which led to it being wildly overbuilt. It had to run high-speed service whisking to a downtown terminus, and also had to intermingle with subway frequencies if that was the tact they took. This is also the reason why the Cabot leads are also so over-designed...they were originally intended for revenue service. The yard for the Cabot maintenance facility wasn't purchased by the T until 1969, after Penn Central had already bought the NYNH&H and was dumping property ballast. This is also why there was no effort at doing a Columbia/JFK platform for the South Shore Branch during design (that didn't come until a 1982 graft-on). The stub-end terminal eventually fell out of favor by the late-60's (about the same time the historical preservation forces had started rallying in-force over saving South Station), but the junction's design was already finished so the only thing that changed was that the stub-end tracks became long shop leads to the newly-purchased Cabot property. The junction remained as originally designed, for either/or revenue service.
In short, the Braintree branch was initially planned to be almost exactly what we consider commuter rail today, just that it's for a specific part of Boston and not the entire metropolitan area. This would also explain much weirdness with the branch's design: the labyrinth Columbia Junction and separate inbound platforms, the long gap between JFK/UMass and North Quincy, JFK/UMass not even having a Braintree platform for years*, and arguably even the car-centric designs of Quincy Adams and Braintree stations. Perhaps it's no surprise that the branch ended up as the longest among all heavy rail lines, by far.
* (You will sometimes hear people claiming the reason why Braintree trains skipped JFK/UMass was racism. While that may have been a factor, ultimately I feel it can't be the main reason, given just how lacking that explanation is compared to F-Line's above.)
So what if none of this happened? Given that the line was built with the intention to become commuter rail, when the regionwide commuter rail concept developed and matured in the following decades, it's likely that the Old Colony lines would have become commuter rail regardless. When that happens, there's no reason to not double track the ROW from JFK/UMass to (just north of) Braintree. And once the Old Colony CR lines take over 2 tracks, the ROW no longer has enough space for 2 more, so the Red Line realistically can't be extended without expensive** modifications. That will put it way further down the priority list, and definitely below BLX (one of the projects that was competing for funding in that era). Thus, the corridor is likely to join my aforementioned list of quasi-transit corridors.
** I'd argue it would be even worse than today. Now, solutions exist to double-track the Old Colony line through Dorchester (as F-Line described here) and Quincy Center station (brief mention here, F-Line may have discussed it in greater detail before). These would enable substantially improved operations satisfiable for Regional Rail standards, despite the remaining toughest single track south of North Quincy, though even that is being looked into by the MBTA. In the hypothetical scenario, however, no 4-track throughout the ROW means no Red Line, as the up-and-running Old Colony CR lines are very unlikely to give up a track.
How much service would Quincy have received in our hypothetical? Even with fully double tracked Old Colony to Braintree, in 2024, each terminal is unlikely to receive anything better than hourly weekday frequencies. This would mean Quincy sees a train every 20 minutes -- which, ironically, is how the Red Line actually operates in 2024. Plus,
any realistic Regional Rail implementation with a moderate goal of :30 on each Old Colony branch can already give 10-min frequencies through Quincy, even more easily so than Lynn/Salem/Beverly and
the Newtons. You don't even have to advocate for it. Thus, the real-world outcome's long-term benefits for Quincy are much less substantial than the harms for Old Colony branches, compared to the hypothetical.
Finally, I want to draw some comparisons to BLX and Lynn. Obviously, Lynn missed out on 5 decades of rapid transit and counting. But BLX to Lynn is very much not dead. It's still one of the most frequently mentioned projects among the community, and its
feasibility concerns, if any, are mostly due to tightened environmental reviews rather than ROW width itself -- in other words, not impossible. On the other hand, if RL Braintree wasn't built in the 70s, it would have been deemed impossible, or at least crazily expensive, every time someone dares to mention it.
(Heck, RLX Arlington is likely feasible and frequently mentioned, yet some people still seem to have the likely false impression that Minuteman completely killed anything beyond Alewife.) This is obviously no consolation for Lynn, but it's at least an interesting thought that their realized present-day situation is at least better than what Quincy would have got in an alternative universe.
(I suepect OLX to Malden vs. Haverhill Line may have been a very similar situation, where a rapid transit extension takes over a rail ROW that causes subsequent commuter rail to be limited to single track, but I'm not sure if the motivation was exactly the same. The difference is that Haverhill trains have an alternative via the Wildcat Branch, so you just need to run :30 trains to Reading or even have OLX take over the entire route. Unfortunately, Old Colony lines do not have an alternative ROW to reach Braintree, other than possibly an El above the highways.)