F-Line to Dudley
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Probably that much, given the need for an additional tunnel under Fort Point Channel. No real point in discussing it, though - the diversion would be far too extreme for the line to work.
Green Line through the Transitway is the only plausible scenario where rail transit comes to SBW. DMUs on Track 61 are feasible, but don't connect to Downtown in a way that makes operational sense.
Yup. And the Transitway is designed specifically for LRT conversion, since the platform heights are all equivalent to Green Line ADA platform heights, the overhead is the same voltage, and City Point used to be a real streetcar Green Line branch running through the Central Subway until 1953. All it would take to run buses and light rail through the same tunnel is rails-in-pavement that hug the platforms as they pull into the stations, the current bus turnouts at the stations likewise rail'ed-in-pavement for passing of out-of-service vehicles, the hangers on the overhead power wire to be converted to the dual trolley pole + pantograph hangers that the Green Line had when PCC's and LRV's co-existed, and for the TT return wire to be raised 4-6 inches (well within the stretch distance of the buses' trolley poles) so the trolley pantographs don't short it out. You literally don't have to touch a damn thing on the stations themselves except hanging green-background signage next to the existing silver-background signage. Combo tunnels of that design are done elsewhere in the world, and Harvard bus tunnel used to be in that setup (albeit without pantograph vehicles) when Cambridge still had a mix of trolleys, TT's, and diesel buses using that tunnel in the 40's and 50's.
I can't even imagine how hard it would be to rip the shit out of those stations for high heavy rail platforms. Much less where those trains could possibly continue once they hit the end of the grade separation at SL Way. You definitely could never get cross-Southie rail service like that old City Point streetcar or the south end of the Urban Ring terminating in the Transitway on a mode incapable of operating in mixed traffic. Southie KO'd their Orange Line El branch off the south junction of the Chinatown portal and the Atlantic Ave. El over 100 years ago. That idea sure as hell ain't ever getting revived on those narrow streets. Nor can you possibly get across the Pike tunnel and the under-pinnings of the Seaport portal of the Ted with a new subway tunnel. It's mixed-traffic branching or bust if you want to retain future options in-pocket for any kind of neighborhood rail run out of the Transitway.
And even if you envision a new transit bore being built next to the Ted eons from now for direct Logan terminal subway service out of SS you'd probably still want LRV's using it so they can likewise pool buses and trolleys together on a dual-mode, grade-separated busway/El or something like that that wraps around the upper level of the terminals. So many public and private transit vehicles need a more orderly and grade separated loop around the terminals that such an option would be too good to pass up for one exclusionary mode and one mode only. Such an ElBusway would be Green Line accessible well before a new cross-harbor bore if they built LRT Urban Ring to Logan sooner. Same thing...rails in pavement, dual-hanger power wire and variable-height return wire if SL1 has any future need to switch back to electric mode over there for a hybrid battery recharge.