I've been interested for a while in how to bring LRT to the Seaport and to Southie, so to that end, I submit my first crazy transit pitch: A loop line through South Station.
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The key contingency in all of this is the sale and redevelopment of the USPS Fort Point facility. Removing it from the picture makes a few interesting things possible.
After leaving Broadway, trolleys would serve the big new development, following a pedestrianized Dorchester Ave and then turning off into a portal where it would connect to the existing Silver Line tunnel by blowing through the bus loop there. Trolleys would serve the existing transitway stations until Silver Line Way, then follow Summer St into Southie and back up along Broadway.
I imagine that this line would be an especially good, nearly 1-1 replacement for the frequently overcrowded 7 bus, bringing a large boost in frequency, capacity, and rider-comfort, and would follow a better alignment by hitting the denser parts of the Waterfront. The weekend boost to ridership between Downtown, the Seaport, and Southie would presumably be quite large. Benefits over the 9 bus are a bit less certain, since crosstown commuters would need to transfer at Broadway in order to continue on toward Back Bay, though at AM peak Broadway is by-far the most common stop for alightings for riders coming from Southie, so this would benefit that set of riders quite a bit while hurting crosstown connectivity.
What would be the fate of SL1/3? Well I imagine that buses would start to turn around at Silver Line Way, not entering the transitway, killing the one seat ride from South Station to the Airport and to Chelsea. However, imagining that Red-Blue comes to fruition beforehand and with SL6/7 better-connecting Chelsea on the other side, perhaps it can be stomached. With a faster trip on LRT to Silver Line Way, a free transfer, ditching the mode-switch, and perhaps allowing the Silver Line to take the State Police ramp into the Ted Williams tunnel, trips from South Station to the Airport and to Chelsea may even become more appealing, in spite of the transfer.
As others have mentioned before, space for a yard is pretty tight down here, so some property-takings are inevitable. The only spot that I could identify is at the Massport site at 85 Fargo St, which you can see I just sort of crayoned-in. It's on the other side of Summer St though, so I'm not sure how that intersection would need to be reconfigured to make this feasible (perhaps via a trench, but is there even enough room for the inclines on both ends to work, and will the water table simply make it a nightmare?)
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Also, does anyone know whether extending the Silver Line tunnel to Dorchester Ave would block NSRL tunnels? If so, my plan should of obviously be regarded as dead on arrival.
So, is a short loop line with only a single in-station transfer to just one other rapid transit service a recipe for low-ridership? Sydney Monorail 2.0? Clearly it does have a few major drawbacks. I can't compare it to any of the plans that call for a reconfigured Green Line to feed a Huntington Ave subway into South Station and to the Seaport, mostly because I don't have the expertise. It would be significantly cheaper, though, and in my mind readily achievable as a medium-term project. But is it the best project for the long-term? I highly doubt it, and there are other projects elsewhere that are more important. Enjoyable thought experiment though.