EDIT: I'm realizing now that I never replied to F-Line's reply upthread. Suffice it to say that I agree all around. The only use I could possibly see for a Reading-Oak Grove short-turn shuttle is to boost frequencies during the off-peak from every ~120 minutes in some cases up to something respectable like ~30 minutes. But even at that point, I'm reasonably sure that the terminal zone is hardly crowded, so there's really no reason not to run those trains all the way into Boston.
This brings up a larger question though -- why are all those Haverhill trains routed via Reading? Is it because there's not enough rolling stock to support separate Haverhill long-hauls (via Woburn) and Reading short-turns? i.e. do you need to use the same sets to do both?
There's also the matter that forced transfers as a streamlining
choice rather than necessity would cause riots across the entire corridor. The Rail Vision did float that very briefly as a trial balloon for fishing Needham Line frequencies out of the gutter by forcing a transfer at Forest Hills...and let's just say that lasted about a nanosecond before never being spoken of again. Oak Grove ≠ Forest Hills on existing overloading on the Orange platforms, but you would still have some very hairy crowding issues for OL boarding at certain peak periods. Remember, they tried this once here during the 2004 Democratic Convention by forcing Haverhill/Reading to short-turn at a temp-reanimated OG platform for a full week to curb the security theatre at the Garden. The results were...checkered at best, with workers either holding their noses or scheduling their vacations for that infamous clusterfuck of a 'lost' commuting week.
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As for why Haverhill is chained? Decision of-its-time, mostly.
Haverhill service via Lowell Line slowly atrophied in the first 10 years of MBTA subsidy from full service to Dover, NH and all intermediate stops in 1964 to cutback to Haverhill + punitive schedule cutback to single rush-hour trip per day by 1967...then loss of the Ballardvale, Andover, Shawsheen, and North Andover intermediate stops in 1974-75. All caused by the old, much smaller MBTA district's very limited subsidy reach beyond Route 128 as the outer towns had to self-fund the full burden of stations and service levels themselves. Failing equipment and further budget shocks led to end of service (then jumping only between Wilmington, Lawrence, Bradford, and Haverhill) in June 1976, 3 months after Newburyport service was formally truncated to Ipswich.
The reversal of fortunes began when the T bought out the windfall of Boston & Maine's passenger ops/stock/property and hundreds of miles of lines in December of that year for a song via the company's bankruptcy settlement. The Legislature quickly voted to expand the self-subsidy reach of the T charter to the I-495 towns and the more-or-less modern geographical reach of the district, while the state bonded out millions of dollars for new rolling stock purchases (first-gen F40PH + Pullman coaches, then the remanufacturing program for FP10 + de-motored RDC coaches). The B&M Freight Main was the first-priority physical plant investment with public funds...and Haverhill, South Acton-Gardner (truncated Fitchburg to Ayer '65, Ayer to South Acton '75), Lowell-Concord, NH, and Methuen-via-Lawrence passenger service were all targeted for immediate expansion by virtue of sharing significant mileage of the Freight Main. Haverhill (12/17/1979), Gardner (1/13/1980), and Concord (1/28/1980) all debuted for service within 3 years. The mainline extensions were for most part built as planned, with only a few station openings (North Andover & Rosemont on Haverhill; West Acton on Fitchburg) not happening and others (Shawsheen; Shirley; Merrimack, NH) trailing late. Methuen on the Manchester & Lawrence Branch and reinstatement of Ipswich-Newburyport were postponed because of not enough budget to tackle poor branchline track conditions. Poor track conditions also claimed the Woburn Branch by '81 as a tactical cut to re-trench to the mainlines. Loss of federal subsidy for interstate running miles claimed Concord and Providence in '81; the crafting of the state-to-state Pilgrim Agreement brought Providence back within 7 years, but NHDOT was subsequently uninterested.
As part of the Freight Main upgrades the Western Route from Reading Station to Wilmington Jct., which hadn't seen any passenger service in >30 years and had been downgraded to unsignalized freight running track, was re-upgraded to mainline-grade. North Wilmington was stuck in at the last second as an excuse-me add (why the platform is so short and spartan to this day). Indicative of how little-used that stretch of track previously was, Woburn St. abutting outbound of Reading Station didn't even have crossing protection and had to be flagged by a crewmember (
NERail photo here shows a conductor flagging the crossing during Week 1 of Haverhill service in '79 because the new gates hadn't yet been installed!).
This was an overt choice for rationing equipment, as the self-propelled RDC roster was 95% kaput and they'd already greenlit the program for rebuilding them into unpowered push-pull coaches. With 3 major-mileage extensions coming online in a one-month span (aided somewhat by start of the 8-year SW Corridor + Needham Line shutdown temporarily atrophying southside equipment assignments to their lowest-ever), they could only juggle these expansions vs. available equipment as linear extensions of existing schedules. That way loco + cab car assignments could stay static despite the massive system expansion, and all they needed to do was lengthen consists with whatever they could beg/borrow/steal for trailer coaches. Reading always retained a full schedule slate, so that was the logical choice to extend. +1 schedule adds like reinstating the Wilmington split just wouldn't work with their equipment reserves...especially with the '79 revival being a substantial full day's schedule slate way greater than the token one-a-day that was whacked in '76. (+1 schedule adds also conspiring against completing the Methuen split.)
Made perfect sense for its era, as the resulting schedules were order-of-magnitude better than Middlesex County had seen in >20 years and overloaded dwells were hardly a problem with quaint 1980's CR ridership. 30 years of growth slowly started to crowd the trains, and by the 'aughts dwells in Melrose and Wakefield started getting bad enough at rush that (in conjunction with all the Bradford Layover relocation talk) there started to be calls for greater re-divergence and lateral schedule trades of more Lowell Line/Wildcat runs for more Reading short-turns to keep the inner stops tolerable. Especially after the Wildcat was thoroughly upgraded in 2001 for the
Downeaster's service debut. Then you had the Andover double-tracking project hit the CIP and the Haverhill-Plaistow extension go into study at the same time the inner-half corridor was talking up the need for "proto-RUR" dense service levels to Reading and the outer-half corridor was talking up the need to knock the end-to-end schedule down more tolerably to an hour. Re-separation of schedules was the common thread to achieving all aims, so it became the more or less default assumption by both halves of the corridor. And once the Rail Vision was convened, that was simply baked in as the uncontroversial Captain Obvious default scheme.
It's time. The mash-up made enormous means-to-end sense 4 decades ago for (1) quickly reinstating service after 3 year absence while the agency was aggressively processing equipment renewal; (2) balancing a three-line slate of aggressive line expansion simultaneously and within equipment margins; (3) delivering better-than-token schedules for those targeted expansions so the growth would stick over time. Despite the unfortunate loss of Concord (which probably wouldn't have happened so soon if Reagan hadn't wanted to make an ideological example of fed transit funding in his '81 Recession budget), history records that they hit a resounding home run with the '79-80 northside revival and got enormous lasting bang for their investment buck. The reason the Haverhill/Reading mash-up
is creaking on its own overload is a direct product of that success, and where that success has led the Purple Line North in terms of next big service pivots.