Longwood can be accommodated by boosting D frequencies...easiest means for that running supplemental E's diverted to Brookline Village on E-to-D surface connecting trackage, then sending them inbound to Kenmore Loop. The Ring, being a radial, never promised--even in its wildest MOAR TUNNEL south-half acid dreams--a one-seat ride from Logan to Southie the back way. The ridership overchurns often...and the quadrants are useful metric of chunking the service because you can specific-target some of the overlaps (such as Sullivan-Kendall-Harvard being a high-demand one-seat for North Shore commuters but not so much for anybody coming anywhere south who's going to make their Red vs. Orange transfers earlier).I think I prefer the deadend at Harvard Sq, and to spend any additional budget for more GL UR at Longwood.
So in this case you aren't hell-or-high-water trying to replace the 66 with a one-seat. The 66 itself will never go away; it just gets relieved of a lot of scheudle-killing overcrowding by turning into "the Harvard-Allston local that then becomes the cross-Brookline local that then becomes the cross-JP local" with way fewer end-to-enders needing to sweat out the clock. The convenience is enabled by simply being able to hop one platform at Kenmore between an incoming Harvard Branch train and anything D. If the loopage-from-E supplement is there the D's Longwood Station would get twice the service while the E's LMA stop would get the wrap-around option, and the micro-targeted doubling of transfer headways at peak (which won't hurt traffic because Kenmore Loop only intermixes with C/D platform berths and the D is far under-capacity) shortens the wait on the platform hop.
Where you can see this eventually going is upping the ante on E enhancements. First the relocation off Copley Jct. and reattachment to Boylston/South End + Back Bay Station. And eventually lighting a fire under extending the Huntington Tunnel to Brookline Village for a full alt-spining of the Central Subway. Because then you'd have bi-directional junctions for service multi-routings at:
- Brickbottom: junction of Central Subway (all thru + Brattle Loop), Urban Ring NW-or-Union Branch, Urban Ring NE, Medford Branch
- BU Bridge: junction of Central Subway (inbound + Park/GC Loop), Urban Ring NW-or-Harvard Branch, B Line
- Brookline Village(-ish): junction of Central Subway (inbound + Kenmore Loop), E/Huntington, D Line, South Huntington streetcar
- South End: junction of Central Subway (northbound, Park/GC Loop, all Brickbottom routings), E/Huntington (incl. all Brookline Vill. routings), Transitway, Washington St. streetcar
We have one major unbreached conundrum with all these neato UR NW+NE renders: what about the south half Ring? We know that cannot be practically built with grade separation, because we'll all be dead eons before there's enough money and cooperation to MOAR TUNNEL all that. And the capacity constraints of on-street BRT or LRT are way more restrictive than the north half. Kenmore-Dudley and Dudley-Seaport are still very good projects even if they have to be BRT on some mixture of reservation bus lanes and striped/mixed bus lanes...but how do we give them an assist on the load-bearing capacity of the Ring? Well, look ^above^ at the exponential linkage being encouraged on the Green Line. That Kenmore Loop bend-back alt flavor of E that's goosing the cross-platform transfers for the diverted 66 riders? Well...if you put some urgency behind the giddayup of that Huntington installment plan the South End junction that eventually takes up the E has multi-direction routing from the E to Dudley OR Seaport. And that Huntington tunnel buildout to BV eventually pipes the capacity. So those Kenmore Loop trains that started out once upon a time as just grab-n'-go spot augmentation are now a major trunk 'circuit' in their own right supporting numerous routing flavors.
So maybe instead of envisioning the south-half BRT Ring SW & SE quadrants being load-bearing like the grade-separated north half, those Kenmore Loop runs turn permanent and scoot Kenmore-Dudley + Kenmore-Seaport as an augmented load-bearing pipe and the BRT-on-striped-lanes crosstown route traces the more faithful 2D arc between Kenmore, Longwood, JP/Roxbury, and Dudley on the surface. And they mutually complement by the alt-spined LRT being able to pipe the crowding at greater frequencies, while the BRT faithfully traces the 'tweener spots in arc corridor and adheres to better schedules from far less overloading. And by working them both in tandem they cumulatively achieve all of the goals that the MOAR TUNNEL killshot would have...except in installments we can actually string together in 25 years of self-pacing instead of likely never being able to build at all.
That's where I think the Urban Ring starts resembling this thread's namesake: "Green Line Reconfiguration". It's the first planks in totally remaking what purpose the Green Line serves. It's not a drop-in replacement for a bunch of one-seat routes taffy-stretched beyond their usefulness. Instead it totally remakes the idea of what a linked trip in Boston is and how convenient it should be by remaking the GL as not a trunk-with-spokes (...or even trunk-with-spokes that we're going to do this 'one' neat radial trick with), but rather a whole dynamic web. We haven't seen anything like that on the service side since maybe the very earliest BERy terminal days...and never ever before scaled to this kind of capacity.
Personally, I think popping our cherry on just the ONE first dip at converting the Grand Junction to NW Ring (+ West starter stub) is enough to ram this point home that it ends up filling the project pipe for another 30 years of constant-churn augmentation to build out the rest of the system. Attractively, none of the individual cogs are in-and-of-themselves all that expensive. Green-Transitway as replacement for Silver Line Phase III is probably the only one complicated enough and monolithic enough to be a $B's megaproject, and that has to be done for a wide-ranging many other reasons than just radial coattails. The other legs discussed have very reasonable costs and are well-insulated from each other for pacing oneself on the installments...and are ripe for micro-targeting and shape-shifting the service plans to fit demand curves that'll be very different on Opening Day vs. 20th Service Anniversary. The momentum gained by starting the first build is enough to carry the momentum forward for the second, the third, and so on.