ulrichomega
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This proposal comes up often, and while seemingly rooted in logic in reality the opposite is true. The worst thing you could do for RL resiliency is to shove the branch split way down to Clayton St. and put Savin Hill on the mainline.
That's because there's still a Columbia Jct.; it's controlling the access to the car supply from Cabot Yard. Since the junction is more or less in an "X" shape with the "X" grade separated at the switches...the four arms of the "X"--subway/mainline, Ashmont Branch, Braintree Branch, and car supply via the non-revenue leads--are virtually impossible to all block/deadlock at once. That's crucial for the line that carries the most passengers on the system. The derailment hitting the signal cabinets was a one-in-a-million crippling shot, but for all the lingering damage from speed/spacing restrictions it did not physically shut down anything other than the Ashmont Branch. It didn't choke the subway's or Braintree Branch's car supply. In the 48 years Columbia Jct. has been there multiple arms of the "X" have been fouled numerous times by numerous types of incidents from minor disablements to major accidents to routine track work to the 2015 Snopocalypse. But the redundancy of the setup made it so that the whole system never crashed...the "X" never lost enough limbs that every single user was completely immobilized and that no trips were possible over at least partial tracts of the line.
If you have a mainline that extends to Clayton St. with the same Cabot Yard tie-in...it IS possible to completely paralyze everything Alewife-Braintree & Ashmont. Split a switch where the mainline and yard leads meet and...bang! Blockage between Andrew and JFK, branches both cut off from Cabot. Ill-time it where there's an imbalance of train supply Alewife-Andrew (maybe this happens on a Sat./Sun. when they're bustituting Harvard-Alewife for track work and that yard's closed), or where the yards are running empty on one of the branches. Now you have an extremely bad situation trying to round up enough cars to provide skeletal service to the nearest crossovers where trains can be turned around. If it's a really bad incident like a derailment that comes off the ground and can't be cleaned up next day PLUS Alewife Yard is empty because of that Sunday night track work shutdown...you don't have more than 2-3 trainsets to run the entirety of Monday subway service. As an acutely citywide-destabilizing event that's much worse and more economically damaging than last week's derailment, even though the crisis would be shorter-lived than our ongoing endurance trial.
In the end it's way less destabilizing to have at least 2 or more legs of the "X" still left standing in-service (even if reeling from a fried signal system) with ability to exchange car supplies than it is to have an outright break. Orange and Blue are capable of having outright breaks because they're mainline only (locations of their primary yards at the midpoints is a hedge against that). But while it hurts a real freaking lot when Orange or Blue go down for the count, not nearly as many trips are at stake there as if it could happen to Red (or Green, but that's even harder to knock out). So with Red being the line seeing the most continued growth and the most queasiness about future overload...we DEFINITELY don't want to mess with the Columbia "X". Just improve it with the next-gen signal system that'll be a lot more operationally resilient (within reason...a one-in-million KO hit from a derailing train isn't exactly a contingency one can easily plan for). They're planning to do just that, and just announced a major acceleration in that schedule.
I've got to search the board Archives for Old Colony double-tracking solves. Will post those next. It won't be cheap, but it's very feasible in Dorchester. Far more a question of will than way. Wollaston-East Braintree Jct. is the truly brutal stretch that doesn't have easy answers.
But surely there are ways around this. Maintain three tracks for the Red Line as an extended yard lead to the Clayton St. junction and ensure the interlockings are designed for redundancy (basically replicate the X but with three tracks. It's basically a flying junction). Redo how cars are allocated during track work. Expand yard capacity at Alewife as part of an Alewife +1 to Arlington. The Orange Line operates in a similar fashion and seems to mostly get by fine.
Your position seems to be that it's impossible for the line to operate without unfettered yard access even in one-in-a-million situations. Surely the rarity of those situations means we can mitigate the downsides in them while capturing the benefits of the most common case?
EDIT: I took a shot at a track map for this and I think I see what you mean a little better now, but I'm still not convinced it's worth crippling the OC Lines for. The concern of one bad crash destroying the Red Line is present, but every other line we have has the same limitation. Honestly the solution is to acknowledge Cabot Yards are in a bad location and build more, redundant car storage elsewhere (or at least make better use of what storage we already have). The primary issue is with north of Cabot, but that's where we desperately need more anyway. Honestly just dig up Thorndike Park for one Winter, build some more tracks under that, and ensure there's allocations for an eventual Arlington expansion. All of the lines other than the Green have weirdly placed yards, and it's something they've had to deal with for a while.
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