Absolute no-go on that routing. It would block the path of the Old Colony/Fairmount lead tunnel into the N-S Link, which has to slip below the Mass Pike tunnel. No one will ever allow a build that kneecaps NSRL like that.
Second, this spot next to where the Bass River gets covered over and where the old South Bay was landfilled is also the
#1 worst 50-year flood-risk spot in all of Boston. The upper half of the Haul Road/Track 61 cut is already a major concern for the 'storm drain' effect dumping an additional canal's worth of water into an already inundated Widett/Cabot area. Put a portal there at that singularly vulnerable spot and all you are doing is building an even higher-capacity storm drain further inundating the also-fucked South End. Required flood mitigation costs for installing multiple sets of flood doors and thicker walls to seal the tunnel against a catastrophic breach will pointlessly double the cost of this routing. It is not "easier" tunneling. It is harder and more expensive, because all the ones that avoid South Bay by sticking north of the highway ramps avoid all the required expense for Netherlands-grade flood controls.
Third, the extra mileage and street-running of the Dorchester + South End jog serves no known demand pattern. It triples travel time from the Downtown transfers to South Station and the Seaport, which is the primary goal of SL Phase III's intended load relief on the Red Line. Chucking the schedule out the window with a distended route running backwards through the more diffuse stops first before hitting SS at the end blunts nearly all congestion mitigation benefits from a Downtown link that hits SS first. If the SS-DTX/Park double-transfer is always going to be 7-10 minutes faster than the one-seat meandering more miles through 6 or 7 lower-demand stops before hitting the main course, people are going to stick to the same Red transfer dance. And downtown dwell times at the big transfer stations will continue to strangle all mobility in town, thus flunking the #1 value proposition of Silver Line Phase III & variants' very existence.
Broadway and its walking-distance catchment already has much faster access to South Station, Seaport, and downtown by Red + transfers. Or just walking. They will not be drawn in nearly large enough numbers to offset the other losses from choosing the meandering routing. Very different story for the Urban Ring since Broadway's on a straightest-trajectory shot from Dudley and the roads best-equipped to trace out the SE quadrant of the Ring. But Broadway being a good Ring stop does not mean it's a must-have for the Seaport/SS-Downtown connection worth warping a map over. The demand audience is empirically distinct and different.
Upstruk, I appreciate the effort and detail you're putting into these renders...but it's falling straight down the trap where the "Crazy" totally sidelines the more important "Transit Pitch" part and just clogs page after page full of dartboard throws. There has to be some sort of demand rationalization backing a proposal. Modifying an ID'd demand corridor into
worse transit because pieces of flotsam on a map may be technically available to stitch together isn't good transit when it doesn't address a question being asked.
And throwing feasibility scoring completely out the window to prove the theoretical technical correctness of a routing in a demand or mitigation vacuum isn't providing a solution, it's practicing bad civil engineering. Civil engineering doesn't exist in a vacuum. Every structure interacts with its interconnected environment, be that mitigation on other abutting structures, the water table in the soil, or safety regulations set by law. And impact their environment on a sliding scale of solveables (e.g. moving a 93 emergency exit hatch 200 ft. down the road) to improbables (e.g. blowing up buildings in tactical nuclear strikes, getting safety laws changed). It's a ledger of benefits and demerits producing a cost-benefit calculation. Which then usefully produces an empirical ranking of Alternatives.
Tape-measured tunnel dimensions do not inform cost, ease, or legalities in a vacuum. When a routing officially studied has been fully-vetted with engineering scoring, the weak scores don't automatically get nullified by drawing a 2D diagram taking a different path...then concluding: "By the absence of the same path, I have an absence of that path's poor-scoring impactors. Thus, this is a superior routing."
No. All you have is an absence of scoring by removing the notion of scoring entirely from the vetting process. Every new render that follows in the footsteps of that fallacy is going to fail for the same reason: you can't determine empirical feasibility by removing the surrounding civil engineering environment from the equation.
