F-Line to Dudley
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RE: the "Red X", also keep in mind that the switch-off allows for more flexibility in variable-length "X" legs than the current branching scheme where longer Braintree has to remain in perfect balance with shorter/stop-denser Ashmont. Such that a Mattapan swallow w/ Milton intermediate by Ashmont would practically have to be compensated with a Neponset/Port Norfolk infill on Braintree to keep similar semblance of schedule parity to JFK where 6-on-6 min. headways = 3 min. mainline headway and straight-up alternating Braintree/Ashmont on the 3's.
You have a little more flexibility with the switch off to alternating legs of the "X" because the line is instead being dispatched much more as two independent end-to-enders that just so happen to cross via Columbia grade separation. So for example, a Phase I Congress St. subway to Community College doesn't have to worry excessively about matching Alewife exactly in schedule length. There can be more bunching/gapping within the 3 min. reference headway at the JFK-proper platforms (which can more than handle it @ quad-track w/ crossovers) on the trading-off maneuver than the immaculate balance that has to be retained for today's service. That aids future planning in:
In short, for ops it's basically like you have two totally different end-to-end color lines run no different than self-contained linear Orange or Blue...like a "Magenta" line and a "Pink" line. Only because of the properties of Columbia Jct. on one trip "Magenta" is Arlington Heights-Braintree and "Pink" is Woburn-Dedham Center...but on the next trip "Magenta" is "Arlington Heights-Dedham" and "Pink" is "Woburn-Braintree". Still run like self-contained end-to-enders agnostic to needle-threaded timing of branch splits, but almost like the "X" is a spinning wheel of changing 'color' lines instead. It's a very elegant layout on scalability and flexibility.
You have a little more flexibility with the switch off to alternating legs of the "X" because the line is instead being dispatched much more as two independent end-to-enders that just so happen to cross via Columbia grade separation. So for example, a Phase I Congress St. subway to Community College doesn't have to worry excessively about matching Alewife exactly in schedule length. There can be more bunching/gapping within the 3 min. reference headway at the JFK-proper platforms (which can more than handle it @ quad-track w/ crossovers) on the trading-off maneuver than the immaculate balance that has to be retained for today's service. That aids future planning in:
- No excessive urgency to know where you're extending ASAP from the northside portal. As mentioned, the studies really have to be thorough to pick the best choose-your-adventure route so rushing it only invites unintended consequences.
- Being able to plan the past-CC route without having to worry about lengthening past Alewife in tandem for schedule balance. Or vice versa. Woburn is longer than Arlington Heights, for instance...but there's no dependency on how you chunk out the builds if Lexington is still a longshot or Winchester Ctr. is an easier Phase I'ing of the GLX Medford conversion than Anderson. This gets even more important when studying out the other crayon-draw build Alts.
- Southern extension possibilities no longer have to schedule-balance Braintree vs. Ashmont. Braintree basically has no further extension possibilities...it's already hit 128, side room along any of the Old Colony branches is wildly unrealistic, density drops way off in Weymouth or Randolph past that stop. But Ashmont arguably still has life in it past Mattapan as 1/2 mile of stationless subway under River St. anchored to a hard Neponset granite seam opens up the quad-track Fairmount Line grading south of Blue Hill Ave. to Readville for side-by-siding of rapid transit through Hyde Park. And possible continuation to Dedham Center on the grade-separated Dedham Branch ROW. This would make Ashmont significantly longer on the schedule than Braintree because there's some very high-leverage new destinations tapped like HP/Cleary Sq. and Dedham C. But that ceases to be any concern when branch schedules no longer have to be pair-matched at all from a single northern main.
- Run-as-directeds are way easier to plug if there is a (temporary or permanent) jarring mismatch in end-to-end'er lengths. For example, if Phase I of the "X" build has to terminate at North Station or Community College, you may be backfilling some headways through JFK to adjust for the longer travel times on the Alewife leg. So maybe for the (more important) sake of just upholding the 3 min. overall churn the pair-offs feature a higher ratio of one point of origin/destination over another through JFK. It's a kludge in the sense that it's less 'elegant' than a straight faithful ALW-ASH / NS-BRN / ALW-BRN / NS-ASH rotation and some pairings may notice a little 'wobble' in end-to-end frequencies. But in the end everyone's still pounded out to rote "3-min. any-train" frequencies from their local platform as most important consideration. If the base frequency is upheld, then a little necessary schedule-churn 'wobble' on the 4-way pairing rotation is a tradeoff riders will happily take.
In short, for ops it's basically like you have two totally different end-to-end color lines run no different than self-contained linear Orange or Blue...like a "Magenta" line and a "Pink" line. Only because of the properties of Columbia Jct. on one trip "Magenta" is Arlington Heights-Braintree and "Pink" is Woburn-Dedham Center...but on the next trip "Magenta" is "Arlington Heights-Dedham" and "Pink" is "Woburn-Braintree". Still run like self-contained end-to-enders agnostic to needle-threaded timing of branch splits, but almost like the "X" is a spinning wheel of changing 'color' lines instead. It's a very elegant layout on scalability and flexibility.