Any reason these aren't "reasonable" ...? I certainly think it's warranted.
1(a). C line continues outbound to BC via Chestnut Hill Ave (no new stops in mixed traffic), AND:
1(b). B line terminates either at Washington Street (not sure they can switch back there? Also means eliminating Sutherland and Chiswick) or at Chestnut Hill Ave.
= Shorter trip to the BC area via the C, and less crowding and opportunity for bunching on the B.
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2. A little less reasonable but still cheap and impactful: B line spur to Brighton Center not via the old A line but via the wide and mostly parking-free Warren Street and along a very short distance in a new median on wide-enough Cambridge Street to terminate at a BC-style switchback loop at Wirt Street (new state police parking to be built over it on a parking deck)
= Great ROI - less than 3000 feet of new track with some street running at minimal impact, to serve a major area hospital and a dense residential/retail neighborhood.
Changing the B's route permanently isn't going to fly when it's been going to BC for 110 years. I'm all for improving it, but it starts messin' with neighborhood forces when fixed routes start becoming less-than-fixed. I think that idea would be quashed in a millisecond.
MassHighway's plan to rebuild Comm Ave. between Packards and Warren and zap the godawful local-express-express-trolley-local lane/median setup is still stalled in design from lack of funding. But if they do that the B reservation gets relocated (at MassHighway's expense) to a much more spacious center reservation with Packards, Harvard Ave., Griggs, and Allston stations getting rebuilt.
There'll be more than enough space here to insert a turnback track at Harvard Ave. and initiate regular short-turns there. And then they have some load-balancing options for helping out the B's schedule:
-- Trade increased short-turns on the inner half with decreased service up the hill + increased service Cleveland Circle-BC. That's not going to inconvenience people up the hill very much because they've seen their headways vary over the years with 3-car consists being spaced slightly wider than 2's. As long as the frequency dip isn't any worse than that it'll be fine. The inner B probably can handle a little more density if a third of the runs get shorted before their schedules have gotten too distorted by the length of the full trip. (Note: in this scenario you probably would have to relocate the Chestnut Hill Ave. platforms to the other side of the intersection so it's accessible from the C. Don't think you want to bypass that stop for a full express since the stop spacing would be much too wide. Although South St. probably has no reason to exist at all.)
-- Stage more run-as-directeds inbound from that Harvard Ave. pocket track to balance the schedules when bunching becomes an issue. The subway can adjust if the inbound arrivals into Kenmore are
predictable. The problem today is the B arrivals into the subway are a total crapshoot, and that makes mixing C's and D's a crapshoot. Lower the schedule margin of error into Kenmore with finer-tuned B arrivals and the subway can handle more total trains. That's how it used to work when the A and B load-balanced each other.
-- Yank more trains out-of-service at Harvard Ave. when shit is hopelessly fucked. Inconvenienced riders get limited to the hill instead of of paralyzing everything, including the BU platforms that can't handle the crush loads.
-- Keep off-peak service running heavier on the B later in the night out to the Allston nightspots to eliminate the localized sardine effect that plagues only that branch...and only that far out.
That's what's going to give them the max flexibility. And it comes close as possible an approximation to your alt scenario of a Union Sq. turnback on Brighton Ave. (which DID exist on the A...there was a seldom-used loop at Cambridge St.). Unfortunately the way they reconfigured the road with left-turn lanes everywhere means reinstating a median trolley on Brighton has to cannibalize parking spots at every intersection to lane-shift the turn lanes one over. And that will draw VIOLENT opposition from the business owners on Brighton, who were the ones most opposed to restoration the first time around (because they had no upside to losing parking with the B duplication a block away). You'll never see trolley tracks on Brighton again unless there's a sea change about re-laying street-running track all the way to Oak Sq.; Union alone won't float it. So a Harvard Ave. B turnback is the easily-implementable approximation that no one will oppose.
Washington only has hand-throw switches, so it can be and is only used in emergencies. And any 2-track only stop will cause more delays than it prevents with the time chewed up changing directions. Every time they try to introduce Brigham Circle turnbacks on the E they quickly abandon the practice because it's not worth the trouble to hurry hurry hurry up and change ends before the next train gets delayed by the dwell time. You really need that center pocket track to stage those kinds of moves on any sort of regular basis. And unfortunately Blandford is too close to Kenmore to have much practical load-balancing effect outside of Sox pregame/postgame. Needs to be situated at the center of the line at a stop where there's a perceptible boardings drop-off afterwards.
EDIT: I also agree with novitiate that BC might like a
D thru-run more than they'd like a C. That becomes a viable option if you do the D-to-E street-running connector track from Huntington to Brookline Village. Bulk up E service by splitting BV runs from Heath runs, then thru-route to Reservoir, then thru-route to BC. You get two "University" branches linking BC-BU and BC-Northeastern, a faster overall BC trip than the C, more service density on the middle-D and inner-E where there's a lot of unused capacity, and free up B + Kenmore capacity to flush more Harvard Ave. short-turns and make the subway schedules more predictable on that end.
Basic load-spreading. It's a good thing. They used to do it regularly, then somewhere along the line forgot how.
