I'd make the Orange line past Forest Hills into two branches. To the north you could make another branch towards the Newburyport Rockport line, through Everett and Cheslea.
Avoid searching far and wide for Orange/Blue branching opportunties, because it's going to be next to impossible to come up with an analogue for Red's dead-on perfect branching that functionally works as well. It took a lot of serendipity for Red to be able to undertake branching elegantly and without kludges.
- Red starts branching in less than 2.5 miles of the CBD transfer stops.
- Add up boardings for JFK + Ashmont Branch + Braintree Branch--i.e. everything >2.5 mi. from the CBD to the south. Then add up boardings for New Harvard + the Alewife extension...everything >2.5 mi. from the CBD to the north. They're within +/- 2300 of a dead-on match (slight advantage for JFK-south). So Red is very highly ridership-balanced on both ends of the CBD, which is how alternating Ashmont & Braintree trains end up being a perfect frequency match for Cambridge.
- This perfect match is not a coincidence, because each branch hosts a major outlying bus hub (Ashmont or Quincy Ctr.), while Harvard hosts a truly monster bus hub about as big as both combined.
- Red is dead-on easy to dispatch because alternating branch trains fulfill demand so well. There's little to no difference in how crowded each branch's train is by the time it hits the CBD, so they don't differ significantly in dwell time drag while they're on the mainline. It's also easy to dispatch because the degree to which the Braintree Branch is significantly longer is well-counterbalanced by its station spacing being proportionately longer and travel speeds being proportionately faster. Alternating trains on shorter but denser stop-spaced Ashmont can hit JFK fast enough to keep balance.
- Extension to Milton + Mattapan does not alter the balance fundamentally, because Braintree in turn badly needs a Dorchester infill stop near Port Norfolk. Adding that extra ridership in keeps pace with Cambridge growth and/or Arlington Heights extension, such that in 50 years it'll still be a straightforward dispatching job of alternating southern branch trains keeping the growing north in equilibrium.
This natural equilibrium is why Red is able to survive and recover endpoint-to-endpoint in spite of its failing state-of-repair and extreme downtown overcrowding. If it weren't blessed with superlative north-vs.-south balance you'd see failures routinely kick the legs out from one branch and/or Cambridge more than any other part of the line, and branch(es) or Cambridge having a much harder time getting back up after a failure. As it is, Red gets FUBAR'ed way too often...but once the FUBAR is over all parts recover more or less in-tandem.
Keep all that in mind when evaluating other HRT branch scenarios. Perfect balance is hard to achieve when the lines aren't set up from Day 1 to allow for robust short-turning and alt-routing to square length and demand differences on the balance of the route. That hasn't been the case in Boston ever since the original Main Line El vision for a mini-Chicago spoke-n'-hub HRT network went stillborn in the 1910's and BERy made the pivot from unpopular Els to stitching those routes together as separate self-contained linear subways crossing at transfers. When the postwar burst of extension studying came out, only Red studied out as a serendipitous match for branching. The others were straight-ahead mainline extensions, or Green Line reimaginings where re-fashioned trunk segments could've been sheared off from the trolley feeder network as linear HRT conversions if they chose.
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You're not going to find a Red-like fit for branching scenarios on Blue, because Blue starts off off-center from the CBD with a demand midpoint distended towards the Maverick area. That means
Lynn-ho!,
Salem-someday! are very bullish extension prospects. And that Red-Blue and that Storrow Drive trade-in Riverbank Subway to Kenmore are attractive prospects to the west (the latter albeit driven by a different kind of politics than your average transit extension).
But take over the D Line??? Or do some westward tunnel-thon under the B&A??? The ridership doesn't scale west of the CBD like it does Revere-north so the frequencies + capacity of 6-car trains are going to weight either way too much for the west or way too little for the North Shore.
Now with a starting point that's already got a mainline geographical skew, what branching could possibly keep that in-balance? Newton Corner + Watertown are semi-thick bus hubs, true, but no such animal exists out on the D Line. Now look at how west end branches demand-balance vs. the North Shore...not simple arithmetic like Ashmont/Braintree staying at near cancel-out level vs. Harvard-north. Much more complicated math, meaning the frequency pairings are likely going to have to be a lot more complicated than just straight-up alternating.
Forget also about branching to Chelsea or the Airport terminals, because Blue doesn't behave like the others at drawing down its ridership neatly right after the CBD. It's got that distended center towards Maverick, a large crater after Logan...but then a steady re-increase towards the end of the line that eventually begats a second North Shore peak screaming bloody murder for extending the mainline to massive Lynn Terminal. That kind of demand layout is unique to Blue on the system. What possible candidate branch could
exactly complement those characteristics ying-yang the way Ashmont's and Braintree's do? Such that no one on Blue is going to end up getting service shorted, deal with divergently different dwells downtown, or have to put up with some very inconvenient ops kludges to maintain a near-balance.
It becomes a house of cards.
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Orange...not much better.
Remember, Amtrak has claim on all 4 tracks south of Forest Hills to Readville and 128 so that route's off-limits. Though even when it was still on-the-board the T was looking at linear extension to Dedham OR linear extension to Readville...not both. W. Rox has similar characteristics to the Ashmont Branch at being shorter/denser-spaced, and Readville 2x longer/faster/wider-spaced. So it's a plausible paper analogue that would be broadly manageable if the ROW were available. Alas, it's not.
Would-be Readville branch has density cavities at Mt. Hope and the area around Metropolitan Ave. caused by the huge expanse of adjacent cemeteries and Sherrin Woods where there's large gaps in the E-W street grid. Hyde Park + Readville have to carry the bulk of the water for that extension vs. the even-keeled ridership curve on the W. Rox branch intermediates, so the Braintree analogue here is sorely lacking its candidate Wollastons at the 'tweeners for aiding the dwell equilibrium downtown. Enough of a wrinkle to give pause at the temptation to scream "Screw Amtrak! Let's subway under their tracks and branch this puppy!" for at-any-cost manifest destiny. At least wait till real-deal regional rail has exerted its full effects on all modes of HP/Fairmount neighborhood transit before waving that particular bloody shirt. FH-W. Rox is a bona fide contiguous corridor, while FH-HP/Readville is more a long-jump between density clusters.
Sullivan-north it's much messier. Where those equilibrium lessons from Red's branching really come into play is when you start trying to come up with Everett/Chelsea branching off Sullivan-north. Bad news abounds there when you throw a major bus hub like Malden Ctr. onto lowered branch frequencies and try to keep the whole works in balance. The bus routes don't pool densely at any one stop on any other fork like they singularly do with Malden, so ops are going to have to scale differently from straight-alternating...at least during times of day when system's bus hubs are slugging highest on transfer crowds. That also affects how neatly the north-of-CBD branches counterbalance the south-of-CBD stops on demand, and which branch train is comparably more crowded by the time it hits the downtown transfer stops. Malden Terminal may be smaller than Sullivan Terminal or FH Terminal south on the mainline...but it
behaves like a typical big Yellow Line terminal on ebb-and-flow. The other branch won't have an analogue that matches behavior. Plus, the branch has to have design characteristics that are in equilibrium with Oak Grove on scheduled time (i.e. the Ashmont = shorter/slower/denser-spaced, Braintree = longer/faster/wider-spaced)...which is a pure-luck crapshoot you're unlikely to get perfectly lucky on with the geography up north.
Impossibly narrow target to make work without induced inefficiencies galore. As long as northeast-quadrant Urban Ring is available on LRT mode hookable to Green, it's not time to be overthinking HRT force-fits for the Everett/Chelsea side of the river. Odds are too underwhelming that they'll play nice with the characteristics of rest of Orange.
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Just avoid the temptation to start digging in for hole-in-one perfection. The T is cromulently anti-perfect, so it's just not going to work unless you find candidate branches that are as BROADLY agreeable as Ashmont v. Braintree v. Harvard-north are at maintaining equilibrium across the board. There's nothing perfect about Red, but its branches have the plus columns on how they balance each other stacked to the nines with demand and ops pro's so the end-to-end balance is robust enough to behave predictably, even in disrepair. Everything else on the HRT map--with possible exception of that Forest Hills fork you can't do anyway because of the spoken-for NEC--requires too much hoping and praying that it'll never hiccup while running...instead of running in broad balance under hiccups like Red manages to do.
