F-Line to Dudley
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16 ft., Interstate-spec. If it's ever any less, FHA regs require putting a height measurement warning sign. None of the CA/T portals have restricted-height warnings like the Sumner and Callahan do, so it's all good. And if it weren't, several layers of pavement would get torn out by default if you're having a dedicated rail ROW with normal rock ballasted railbed instead of street-running rail. Rail tunnels don't need anywhere near the multi-foot layering of pavement and reinforcement on the floor that a road tunnel does.
The single-lane EB tunnel is plenty wide enough for 2 trolley tracks if you cannibalize the catwalks on either wall, which are unnecessary for LRT. It's got decent-size shoulders on it in addition to the catwalks. Since the other bore is 2-lane you still have unused tunnel capacity to spare for other purposes.
This is where I strongly disagree. A direct connection from Boylston to SS gets you from North Station to South Station in 6 stops. At most 7 if it's a Marginal St. wrap-around with Ink Block infill. And every single one of those stops except Ink features an inter-line transfer, with 10,000+ riders at all except Ink because of the transfer utility. The backdoor route, on assumption that you're not just running express from Tufts to SL Way, is anywhere from 10-12 stops, same as Brookline Hills to North Station. With a huge ridership crater at the string of intermediates between Tufts transfer and SS transfer. The longest SL III replacement routing through the South End is ~2-1/3 track miles from North Station...approximately the distance from North Station to Hynes. This backdoor route is 2 miles longer, the equivalent of North Station-Brookline Village (that Brookline Hills comparison is not far off at all). At such longer distance with many more stops the headways are going to be more diffuse. And the overlap with longest-distance SL1 arrivals and longest-distance trolley arrivals at the portal is far more variable for dispatch to time vs. trolleys that start intermixing when the Silver Line is at end-of-run looping, or vice versa at SL Way where trolleys are end-of-run looping. The variability is going to require more spacing of trolley vs. SL1 headways for bunching/conflict prevention, further reducing the service density on this backdoor route. It's going to be more like the C or E than "halfway-to-mainline" service levels. Better than today's Silver, but not nearly dense enough for the primary demand of load-spreading Red around downtown.
That's an order of magnitude poorer travel time and frequency from being faster than the double-transfer dance at SS+DTX and SS+Park. A majority of riders needing to bounce around downtown are going to keep doing what they're doing, and Red won't improve. The load relief comes from a niche audience at the mid-line intermediate stops, who will no doubt appreciate this. But it doesn't do meaningful enough work for the prime downtown circulator demand seeking immediate direct access to that murderer's row of transfers. That overcrowding and dwell time decay on Red is not going to reverse itself, and any reduction in headcount is going to be inconclusive and short-lived compared to taking the direct routing.
It's not asking the same demand question. That's why it's spurious logic to pitch this as meaningful partial answer. It is closer in actuality to the big misjudgement that the Silver Line made to justify its modal choice: that Dudley Sq. to Seaport via Downtown was somehow a service pattern that beckoned a build because it just so happened to look like a stately color line on the map. It wasn't; it was a niche nice-to-have for a handful of people while the two halves of the line kept wholly separate demand audiences that rarely intermixed. The ridership overturn at Boylston projected to be near-total, with Seaport-Downtown the orders of magnitude bigger half than Washington. It necessitated that performance-killing mid-line loop because the niche demand wasn't compatible with the primary demand and apportionment of headways didn't work with the asymmetry.
As I said, that's an excellent path for the Urban Ring where the radial circulator links neighborhoods and outer transfers first, downtown last. But blurring the lines between SL III's downtown ground-zero demand and the UR's radial demand by shotgunning partial pieces together out of engineering expedience makes the same error in judgment that the Silver Line did conjoining Washington with the Transitway out of engineering expedience. If not quite as badly, then at least a lower-case version of history repeating itself with the same logical error. It's poor allocation of resources resulting in bad(-er) transit to start with a leading answer that has to kludge together fringes of the demand question for self-justification. That's not useful here. There is a screaming-obvious demand question being asked of SL III and its alternatives: move those hoardes out of South Station and distribute them immediately to the transfer points so Red and the DTX + Park congestion on the Red platforms can save itself, and in turn uncap Seaport mobility BOTH on the one-seat circulator AND on Red. Full-stop...nothing else matters unless the build engages that head-on. Taking a swing from Seaport intermediates through Broadway at the triple-junction of residential Southie, Dorchester, and the South End is the job of the Urban Ring: radial, neighborhood-to-neighborhood, transfers outside the CBD. Blurring the lines isn't clarity; it makes for muddled solutions that don't do their job nearly as well as hoped. We deal with that frustrating gap every day on Silver; we'll be dealing with it on the backdoor meander when headcounts on those overstuffed downtown Red platforms don't inch down enough to matter.
