This would give commuter rail passengers a transfer at west station to take green to kendall. It would also give riders from downtown a ride to kendall as well with transfers from blue red and orange. Red already goes to kendall but the gj green line would have multiple stations throughout cambridge giving more options. There would also be a transfer in kendall from red to green to travel to the other stations or west station.
I've not perused the latest in the smoldering dumpster fire that is the West Station project, so I'm not sure if the current plans even allow sufficient room for CR+LRT, so if someone knows the answer to that or can point me in the right direction that'd be appreciated.
West's status serving the Framingham/Worcester line means it's value as a transfer node from the CR mode is always going to be limited compared to the transfer stations that serve multiple lines (North, South, BBY). That's not a bad thing, but it does mean that it's harder for it to be load-bearing as a reason to build a transit connection. This is especially true where the benefit for Kendall access specifically is relatively limited; it's already a straight-shot transfer Worcester CR-to-Red at South to get to Kendall, though there would be improvements to Cambridge access beyond Kendall specifically from a GJ LRT connection at West Station. (Perhaps more valuable to the system overall is the fact that those riders would presumably be taken off Red's overloaded downtown segment, a nice relief valve for as long as Red has to handle itself and the Silver Line passengers who just get dumped there by that monstrosity never being properly completed.)
Probably wouldn't see that much ridership shifting from Orange, and only some from Blue (very little if Red-Blue to Charles gets built, more if it doesn't) and some from Green (specifically from whatever branch or branches got sent to the GJ) because the current pattern is so reliant on Red that it'd take quite a bit to overcome that built-in inertia, and the Green Line's overlapping service patterns means that a GJ-specific car through the Central Subway might well be a longer apparent (or actual) wait than Red at DTX or Park (as well as a likely longer ride). Northside CR riders (except Fitchburg if we had proper CR-subway fare integration) would benefit.
The connection with the B branch could be added in later after the line gets up and running, but this would be very simple to implement at first. This is what I think should be done with the gj line since an elevated line has buildings in the way overhead, a tunnel below gj has the red line tunnel in the way + fill and especially high cost. An electrified commuter train option also doesnt give it a route through downtown, and the trains are longer, slower, etc. At grade crossings wouldnt be bad with 2 car green trains passing quickly.
The state actually studied (non-EMU) Commuter Rail from Worcester over the Grand Junction when the guy whose name I can't remember from out that way was lieutenant governor. The results were...unfavorable. The grade crossing impacts were brutal on the RR mode because of FRA-mandated railroad priority, several of them physically could not be eliminated on that mode (Main Street probably can't be eliminated on any mode), and the resulting schedule possibilities were atrociously poor. I think the fact that fairly-cheap,
relatively easy GJ revival was not taken up despite basically being a freebie for the then-lt. gov tells us enough about just how unsuitable the GJ is for passenger service (let alone one trying to emulate transit service).
A Green Line branch, as LRT, doesn't have the same crossing impacts (simple priority, assuming they don't botch that as badly as they have every other time it should have been done on the system, will suffice) and its ability to handle significantly-steeper grades makes the prospect of outright eliminating a couple of those crossings considerably more feasible. So, I think you're entirely correct to revive the argument that LRT is the clear best-fit option for GJ transit.
The one significant issue I have with your proposal (beyond quibbles about West Station proper) is operational. Lechmere to West Station via the GJ is very roughly equivalent in length to the Medford/Tufts branch of the GLX, which means we'd need yet more cars and extending yet another branch further. I'm not particularly opposed to
that, I just don't think it makes sense to do it when the B-branch connection (while harder) gets a
lot more value (especially if the northeast quadrant of the Urban Ring to Everett/Chelsea/Logan ever got built as LRT). I'm not saying that it shouldn't be done, but I do wonder whether it really makes sense as a standalone rather than having, say, the ability to access West and run back through to Kenmore and the Central Subway. (It's a good proposal, don't get me wrong, I just don't personally think that doing an easier project is a better choice than doing a harder, but more flexible and therefore beneficial project.)
