That's where a proper study would be able to determine the costs of each option. A full portal build of the N-S Link or a partial portal build with connections for HRT at the north and south sides. Another point on HRT is that with all the new regional traffic, MBCR, NHDOT, etc., the N-S Link brings (mostly from the Lowell Line) slots for the Western Route (current Haverhill Line) start to get cramped, like the Needham Line is currently cramped by the NEC. You won't be able to route HRT level headways to Reading. That's when outright conversion of Reading to Orange starts to look more attractive/cost-effective. Is it more worth it to have redundant electrification along the Western Route to Oak Grove, or just continue the Orange Line to Reading and call it a day?
Yeah.
One of the things that will force this conversation is cost of the extra portals. That's about +1 mile of extra tunneling just to hook the Old Colony and Fairmount up to the thing. A bit less for the Fitchburg fork, but if everywhere-to-everywhere regional rail is the goal and these extra appendages are billions more unto themselves, it calls into question just how much $$$ pain is tolerable for complete hegemony across the mode. That how-steep-a-hill-to-push-the-boulder consideration.
So it's very likely that the base build is just going to have to concentrate on the NEC-north routing and leave wall cuts for adding the Fitchburg, Old Colony, and Fairmount portals later. That's reality. You won't get everywhere-to-everywhere until 15 years after it opens, if that. But what destinations does that truly prevent? Ayer-Wachusett is accessible out of Lowell via the Stony Brook Branch at only 5 miles longer a trip than the Fitchburg main. Run a limited-stop express stopping only at Anderson, Wilmington, Lowell, some intermediate on the Stony Brook, then Ayer and all regular stops and it matches or slightly beats the existing Fitchburg schedule, because Lowell will presumably be pushed to 90-110 MPH by this point. Middleboro-Hyannis are accessible via Stoughton-Taunton-Middleboro. Run an express skipping most to Taunton Depot and it can sync times with an all-stops Middleboro run (this may be necessary way before the Link to grow Cape ridership while sidestep the Old Colony capacity pinch).
What does that truly omit from a thru-downtown one-seat? Fairmount, Greenbush, Kingston/Plymouth, Belmont Ctr.-Littleton, Holbrook/Randolph-Bridgewater. What % of systemwide commuter rail service and ridership is that? What % of the Link's total cost does bringing in that small minority of CR ridership carry? I don't see any way with the project's total price tag that they can justify including these destinations in the base build. It's either a 20-years-later addon, or none at all. That's a pickle for Fairmount, but how big a loss is this really for the others? It's not like they lose service, as the surface terminals are not getting abandoned. If anything they'll be just as busy with increased frequencies on all lines filling up the slack for what gets diverted underground.
This won't be SEPTA where the terminals go away entirely. We're not SEPTA; SEPTA has no systemwide equivalent to the 495-oriented lines or intrastate intercity jockeying for slots with its 128-equivalent EMU service. SEPTA didn't serve nearly as many masters as MBCR even before it made that disastrous decision to whack all its diesel routes. In practice there won't be absolutely flawless 100% integrity-of-concept in CR thru-running. Set aside any Transit OCD notions that this is a one-seat magic bullet. The highest-demand lines and most-favored communities will get the thru-running spoils. Blanket, incremental multi-modal enhancements are going to be the only way to address localized needs and adapt to the wide-ranging changes a megaproject like this causes. This relieves NONE of the pressure on the rapid transit and bus systems to circulate better through the urban core and push their trunks further out towards 128. If anything it puts crushing additional pressure on all modes to hurry up and start filling in all those HRT/LRT and circumferential builds they've been deferring for 20-70 years.
If Phase I Link is all that's swallowable off the bat, that forces thorough examination of what add-ons provide the better value? Does the Old Colony need thru-running at all if a Red-forked rapid transit line ran through the northside? All Greenbush, Plymouth, and Bridgewater riders can transfer to Red at Braintree, JFK, Quincy Ctr., or SS to get north. What are they being deprived of? Does Waltham need the best thru-running forever DMU money can buy, or do they need a real rapid transit line? GLX is sitting there on-trajectory from Porter (or Red from Alewife, though Lexington's probably the better continuation), and the Fitchburg + Central Mass ROW's can fit both modes out to 128. They're going to be waiting 20+ more years until thru-running DMU's are routable from there, same timetable as the whole tortured study/design/build process of a GLX-like project. Do the lack of other plausible alternatives at Lincoln, Concord, West Concord, South Acton, and Littleton alone justify the price tag of that Fitchburg portal?
And can the Fairmount--the most vexing thru-running deferral--be as-well or better-served having a direct Urban Ring connection at one of the Dorchester stations and a direct Red connection with some pain-limited tunneling near Mattapan Sq.? (Note: solid granite bedrock down by Milton-land is excellent deep-boring material). Or does outright side-by-side Red service @ Fairmount and Readville stations serve Hyde Park and Mattapan infinitely better than the bestest DMU? The Fairmount corridor is 'pure' neighborhood-to-neighborhood transit; the locals transfer to/from buses at each stop more than any of the other DMU line candidates. Does a combination of HRT touching Mattapan and/or Hyde Park and thick 'net' of transfers through Dorchester serve the corridor's needs better than the $B's for an extra mile of RR tunneling and thru-running perfection? Pretty lines on a map don't necessarily correspond to where some car-free Dorchester resident needs to go on every trip.
I don't know how this will play out. We'll probably all be collecting Social Security before the
minimum Link build is getting finishing touches. But if they're faced with cost constraints that are going to force decisions on whether to defer the extra portals, it's quite likely going to be a commuter rail that for the first 20 years omits that minority % of stops from thru-running. A build that will not give everyone, everywhere a bite at the whole cake, all the frosting, and the cherry on top from Day 1. And maybe even leaves the Track 3 & 4 berths in the tunnel vacant for a later day, needing to choose a more gradual ramp-up of thru-running capacity. So then the study focus has to shift to what's the fastest and most cost-effective way to get those surplus-to-requirement destinations jump-started. Is it going to be waging the funding war for the rest of the RR build? Or is it going to be a rapid transit build-out to trim costs on the Link entry points and spread the wealth elsewhere building out rapid transit to the outer neighborhoods and 128? And concede to the fact that Lincoln-Littleton, Holbrook-Bridgewater, Greenbush, and Plymouth may just have to live with the same terminal runs at (the horror!) greatly increased frequencies. That Morton St., Talbot Ave., Four Corners, Uphams Corner, and Newmarket are just going to have to live with increased terminal service and the bus transfer frequencies of their dreams (egad!). While Mattapan and Hyde Park have to suffer...
suffer!...through an Ashmont Branch extension running every 3 minutes.
How is this a bad thing?