F-Line to Dudley
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Blue Line cars fit Green Line dimensions everywhere except possibly Boylston curve proper. And only possibly, because Bowdoin curve is just about the tightest HRT curve in the world. Widening out the curve would be more a necessity because of the excruciating speed restriction it would induce rather than the cars either not physically making it or needing more than a negligible smidge of extra breathing room to make it. Old Orange Line cars (slightly smaller than today's OL cars, but bigger than Blue) did go Haymarket portal to Pleasant St. portal from 1901-1909, and all subsequent tunneling (Kenmore 1919, B/C branch portals 1932, Huntington subway 1940, D portal 1959, GC-Haymarket 1963, North Station 2004) were each built with future-proofing.
The problem is simply what you'd do with all the branches you wouldn't be able to run that way. When BERy first toyed with the idea there was still a dense enough streetcar network where the C and E looping at transfer stations provided adequate scale connected to the rest of the network, and losing the B from Packards Corner to Chestnut Hill Ave. in lieu of a single HRT subway through Brighton Center wouldn't have been a crippling loss because the 66 trolley on Harvard Ave. would've taken up as cross-Allston load-bearing route from an easy HRT transfer on Harvard @ Brighton Ave.
That scale is lost and really can't be built back, so you lose too much in a conversion to be able to stitch the orphaned branch parts back together. You also lose the ability to run all that many branches period by trading wider train spacing for longer trains with HRT conversion, and can't negotiate grade crossings which are probably going to exist in some hard-to-eliminate residual spots on the Urban Ring (Main St. + Broadway, Cambridge; 6th St./Arlington St., Chelsea; the Upper Falls-Needham Center section of the Needham Branch if external forces with Orange and commuter rail force Town of Needham's service to get traded between modes; etc., etc. You also have some major issues lengthening the subway stations to 6 cars if you want to take advantage of the full capacity of HRT.
In short, there's just too many compromises in an outright 100% conversion that take a half-dozen extra improbable-difficulty megaprojects to compensate for all the potential transit loss on the branches. If you read back in this thread the evidence is pretty conclusive that with full state-of-repair and load-spreading trunks that light rail not only can perform hugely better than the sickly Green Line does today...but that it's massively scalable in ways that HRT isn't. HRT works best as linear extensions out to Route 128 in each direction, preferably with minimal branching since the 3 lines aren't interoperable, can't easily be wrapped around each other in trajectories logical enough to demand patterns to force-fit them as interoperable, and don't have the huge track miles of express tracks that NYC Subway has enabling a lot of run-thru interoperability. So we have to pick our modes by role. Having a robust LRT network to scale off of lets Boston develop more crisscrossing transit density so the capacity of the HRT lines doesn't have to be warped into an ill-suited role of branching here, branching there, branching everywhere.
Now, as a load-bearing companion to the Central Subway you can absolutely leverage a future teardown of Storrow Drive--or at least the eastbound carriageway while westbound is left as a two-lane low-capacity park road--with a mandatory transit trade-in requirement, extend the Blue Line from Charles MGH to Kenmore, and have a full-on second trunk freeing up the heaviest-ridership part of the Central Subway for demand originating elsewhere. That was a real and almost-enacted proposal a century ago, and the Storrow-for-transit trade-in is the one avenue that could realistically revive the proposal (graduating to build...another matter; serious debate...yes, potentially). You would then have options for extending Blue past Kenmore. The D seems obvious, although--caveat--the branching AND grade crossing situations are likely to rear their head with the Needham Branch when NEC congestion and the need for eventual Orange extension to the neighborhoods past Forest Hills tag-team to squeeze Needham off of the commuter rail mode. You'll never totally be free and clear of those HRT vs. LRT compromises, so pick your battles judiciously. There's plenty of linear expansion potential on all 3 HRT lines to whet one's appetite, and plenty of ways to reimagine the LRT network keeping it--and maximizing it--as LRT. There'll never be a loss of choices for using every mode to its natural advantages. They're all tools in the transit toolbox.
The problem is simply what you'd do with all the branches you wouldn't be able to run that way. When BERy first toyed with the idea there was still a dense enough streetcar network where the C and E looping at transfer stations provided adequate scale connected to the rest of the network, and losing the B from Packards Corner to Chestnut Hill Ave. in lieu of a single HRT subway through Brighton Center wouldn't have been a crippling loss because the 66 trolley on Harvard Ave. would've taken up as cross-Allston load-bearing route from an easy HRT transfer on Harvard @ Brighton Ave.
That scale is lost and really can't be built back, so you lose too much in a conversion to be able to stitch the orphaned branch parts back together. You also lose the ability to run all that many branches period by trading wider train spacing for longer trains with HRT conversion, and can't negotiate grade crossings which are probably going to exist in some hard-to-eliminate residual spots on the Urban Ring (Main St. + Broadway, Cambridge; 6th St./Arlington St., Chelsea; the Upper Falls-Needham Center section of the Needham Branch if external forces with Orange and commuter rail force Town of Needham's service to get traded between modes; etc., etc. You also have some major issues lengthening the subway stations to 6 cars if you want to take advantage of the full capacity of HRT.
In short, there's just too many compromises in an outright 100% conversion that take a half-dozen extra improbable-difficulty megaprojects to compensate for all the potential transit loss on the branches. If you read back in this thread the evidence is pretty conclusive that with full state-of-repair and load-spreading trunks that light rail not only can perform hugely better than the sickly Green Line does today...but that it's massively scalable in ways that HRT isn't. HRT works best as linear extensions out to Route 128 in each direction, preferably with minimal branching since the 3 lines aren't interoperable, can't easily be wrapped around each other in trajectories logical enough to demand patterns to force-fit them as interoperable, and don't have the huge track miles of express tracks that NYC Subway has enabling a lot of run-thru interoperability. So we have to pick our modes by role. Having a robust LRT network to scale off of lets Boston develop more crisscrossing transit density so the capacity of the HRT lines doesn't have to be warped into an ill-suited role of branching here, branching there, branching everywhere.
Now, as a load-bearing companion to the Central Subway you can absolutely leverage a future teardown of Storrow Drive--or at least the eastbound carriageway while westbound is left as a two-lane low-capacity park road--with a mandatory transit trade-in requirement, extend the Blue Line from Charles MGH to Kenmore, and have a full-on second trunk freeing up the heaviest-ridership part of the Central Subway for demand originating elsewhere. That was a real and almost-enacted proposal a century ago, and the Storrow-for-transit trade-in is the one avenue that could realistically revive the proposal (graduating to build...another matter; serious debate...yes, potentially). You would then have options for extending Blue past Kenmore. The D seems obvious, although--caveat--the branching AND grade crossing situations are likely to rear their head with the Needham Branch when NEC congestion and the need for eventual Orange extension to the neighborhoods past Forest Hills tag-team to squeeze Needham off of the commuter rail mode. You'll never totally be free and clear of those HRT vs. LRT compromises, so pick your battles judiciously. There's plenty of linear expansion potential on all 3 HRT lines to whet one's appetite, and plenty of ways to reimagine the LRT network keeping it--and maximizing it--as LRT. There'll never be a loss of choices for using every mode to its natural advantages. They're all tools in the transit toolbox.