Let's be frank...they don't really have a choice. The branchline slowpokes of the NEC fare worst amid the RUR and Amtrak increases through the SW Corridor bottleneck. Needham flat-out doesn't have a future...the need for it to hug a single bi-directional track to hit both the Ruggles and Forest Hills platforms is the noose that's now snapping tight and leaving it the gimp of all services. At least inside the tunnel if you could level things to a flat T vs. AMTK world where everything Amtrak skips Ruggles, everything T hits Ruggles, and crossover games are played accordingly...you can make a tri-branch CR schedule with Franklin work as there's no difference in how it plays the "to cross over or not to cross over" rules vs. any Providence or Stoughton slot.I don't disagree that there is a need for this, but I'm skeptical that it's feasible to express through stops on the Fairmount Line, both operationally and politically. Back in the day (2015), there were a couple of express runs on the Fairmount schedule that did Readville-South Station in 23 minutes. By contrast, pre-pandemic it took an NEC train that same amount of time if it stopped at Back Bay, Ruggles and Hyde Park; if it expressed to Back Bay, it could swing it in 18.
It seems hard to imagine reliably sliding an express run in between 15-minute Fairmount All-Stop headways. And if you start adding in more intermediate stops, then aren't you falling away from being competitive with the NEC routing?
That being said -- the current (pre-pandemic) Fairmount All-Stop schedule timetables the journey from Readville to South Station as 30 minutes even, which is significant, but not earth-shattering. So I'm not sure trying to plan overtakes is worth the trouble.
Different story altogether once you get into fresh-minted quad-track territory outside of Green St. The line is straight enough for Amtrak to briefly hit triple digits on the 4 miles to 128, and the T (depending on whether Hyde Park or Readville are in the mix) sustaining 80-90. This is where Amtrak really makes hay on its traffic priority by being able to brute-force overtake on its own pair of center tracks without there needing to be crossover games (because crossover switches do nick up high-speed running just enough to matter). And assuming 128 is redone as a quad-track/two-island station with T on the outside and Amtrak in the middle...that traffic separation basically carries through to Canton Jct. split regardless of whether the rock cuts south of Westwood ever get widened for >2 tracks. But the key here is that there can't be unnecessary crossover games if this is the 4-mile stretch you want to target for brute-force priority-setting overtakes. That's where Hyde Park becomes a giant question mark if they can't rebuild it as a 2-side platform/2 center express track station and have to "squish" it on a one-side island (as NEC docs seem to indicate may be the issue). But it also means Franklin is now the odd-man out because it does have to play crossover games cutting across all 4 tracks between HP and Readville to make its split/merge.
With Rail Vision traffic modeling there will no longer be enough open slots projected against Providence/Stoughton RUR frequencies and projected Amtrak growth for the full schedule slate of Forge Park to cut across. Multiple slots on the day's schedule are conflicted right off the bat by the change in NEC traffic management.
The only way to *guarantee* having any slot available is interlining via Fairmount. Now...this would not be the stet 9/6/2020 Fairmount Line. For one, the Readville platform moves up 200 ft. into a double-track island and 2000 ft.'s worth of single main gets completely doubled-up through the station instead of going through yard limits. Fairmount's platforms also get raised so the auto-door coaches can work to/from Readville before going to manual + traps @ Endicott on the freight clearance route. And where right now there are only 2 crossovers on the entire line (Southampton Yard before Newmarket, and between Four Corners & Talbot Ave.) additional ones would be added for a rate of 1 crossover per every 2 stations matching the reference layout for the Worcester Line in Newton for supporting Riverside Urban Rail amidst its service layer cake. Right now overtakes are not really possible with any acceptable precision for a line tasked with carrying more than 1 full-time service pattern, and thus the couple alt-routed Forge Park runs today don't try to play hopscotch at all. They just grab a gap and run up to the taillights of the next-nearest train, which is hardly a time-optimized way of running. With the extra crossovers on the infrastructure, they'll be able to do the precision-overtakes that a co-running Forge Park schedule demands.
Now...as for the NEC, that alignment will still be available *some* of the time during opportune gaps. So I very much doubt Forge Parkers have seen the last of Back Bay Station. It's just that the Rail Vision has already modeled NEC ops and deemed Franklin enough of an odd-man-out that you can't base an all-day reference schedule on running that way. So maybe the :30 minute FP turns run by Fairmount two-thirds of the time, NEC one-third of the time...NEC turnings a little more irregularly spaced since on the spread since they're fishing for conflict-free slots. It's hardly all-or-nothing. But if you want :30 to FP, you're going to have to concede a whole lot of NEC usage. Simply no two ways about that. So let's say that they clock-manage the Fairmount skip-stop jaunt and the NEC grab-n'-go slots to make equal time. Fairmount gets backfilled with an extra short-turn while FP is using the NEC, and schedules to Readville match routing-by-routing so the :30 churn stays exact.
Slightly kludgy...yes. But nobody promised a zero-kludge Rail Vision. This happens to be the one regular-churn route on the whole system that doesn't compute out with perfect mapmaking consistency. I guarantee for the frequency increases that there aren't going to be riots on the streets of Norfolk that 2 out of every 3 times they're going to have to make do without Back Bay. They were too busy thunderously applauding at "30-minute all-day service".