^ Let me see if I can beat
@Teban54 on the reply here
can someone explain to me *why* a LRT green line connection under essex to SS and the transitway doesn't work
First off, welcome to the board! I'm glad this is interesting, and I'm touched to hear that my maps contributed.
An Essex Subway is one of those things that seems workable until you start looking at all the things you would need to mitigate.
1)
Width: you need one or two stations to provide transfers to Boylston and Chinatown stations. The logical place for one
is in the block between the two. But, the buildings there are barely 50 feet apart across the street, which might barely be wide enough for a surface level station, but is too narrow for a single-level subway station. So, we say, no problem, just stack the platforms on top of each other. But that leads you to...
2)
Depth changes: now you have one of your platforms all the way down at Level -3 (below the Green & Orange tracks, and then below the first platform). That already means that you are digging a deeper tunnel (see below), which makes it more expensive.
And now you need that tunnel to rise up from -3 to fit over the Big Dig Tunnel and then be able to hook in with the transitway. IIRC, that is physically doable, but much more complicated, which is a problem on a project that will need lots of...
3)
Building mitigations: Essex St is old, and the buildings along it are old, and a new subway -- particularly one that goes 3 levels down -- will need to dig away right next to those old buildings, down along and probably deeper than their foundations. Each building will require its own set of measures to keep it from collapsing during and after construction, which will be expensive and complicated. Plus, as an old street, Essex likely has unmapped utilities and other surprises waiting for us down there. And that's not even considering the special mitigations needed for...
4)
Underpinning Boylston and Chinatown stations: by digging under the stations, you'll need to find a way to hold them up while you dig and keep them held up after you finish. More time, more complexity, more money, particularly given how old these stations are. And it becomes even more painful when you realize that you need to do it for two levels' worth of new tunnels. You'll also have to figure out how to build a transfer connection down to the new subway (including all the way down to Level -3), which... I don't even know how you would do that at the existing Boylston station -- those platforms are pretty constrained and don't even have a free transfer between inbound and outbound
I'll be honest, I needed to go back and double check myself before answering, which speaks to one of the annoying things about the feasibility of the Essex Subway concept: there's no single fatal flaw, but rather a bunch of problems stacking up on each other until the whole thing becomes impractical. (F-Line will argue that the underpinnings were the fatal flaw, and I don't necessarily disagree with that, but my point is that even if you can find a way to solve that particular problem, there will still be multiple others fighting you.)
And, also frustratingly... it's not that the Essex Subway is absolutely or physically impossible to do. Given enough money and time, yes it probably could be built. But the question is, how does it compare to potential alternatives (including no-build)? And by extension, would the money & time spent on it be more effective if spent elsewhere?
It's definitely an unsatisfying combination of, "Yes this could be done in theory" and also "It would be so expensive and difficult that it'd be hard to justify when compared to other projects". And also yeah, it looks
really nice and clean on a map. But it just becomes harder and harder the more you look.
Does that make sense? Sorry for the TED Talk (though I imagine you knew to expect that by now, lol).
I know the agreement here seems to be the Gold Line route, but to me, it does seem to be a problem to not have any sort of B line/C line/Kenmore and onwards connection to South Station/Seaport.
This remains, as ever, the annoying fly in the ointment. Because, yeah. It's an unaddressed need.
@Teban54 has an idea for a modification/elaboration of the Gold Line proposal to enable Kenmore <> South Station -- I think his is an interesting proposal and also would bring its own set of complications.
For my part, I've tried to address this in two ways. First, building a direct transfer between Copley and the new Gold Line platforms at Back Bay. This would be in the neighborhood of 800 feet, which is long, but IIRC is comparable to the Orange <> Blue transfers at State.
The second is to
just straight up extend a BRT surface route along Essex. Like, South Station actually is pretty darn close to the Green Line -- about 2500 feet between South Station and Boylston. That's just not that far. With some good BRT infrastructure in place, Kenmore riders can get the 2SR to the Seaport that most other riders across the system would also enjoy via transfers to the Gold Line, SL3, and a Congress St busway.