It was discussed in the 2003 PMT (pg 5C-32 and 5C-33, pg 158 in the Report PDF). Vanshnookenraggen also included it on a couple of his early maps way back when.I don't think there's ever been an official proposal for the Blue Line to take over GLX, though it's been thrown around on forums for a while. The idea of Red-Blue is very new - even into the 1970s, extension proposals usually turned it south to Park Street - and the 1986 study had station designs that would have precluded any further extension:
Yeah, the fact that Red-Blue is relatively new is wild to me, all the more so since there was a hot second (well, a few years) when streetcars ran through Bowdoin up on to Cambridge Street and then over the bridge. When the East Boston Tunnel was converted, that route wasn't really bustituted or replaced, so I guess it just sorta faded from collective memory?
@Teban54, thanks for your interest and questions! I'm glad what I've written has been useful so far, and yes, I think you will find the next posts interesting. @Brattle Loop walked through a number of the challenges that Blue-under-Huntington would entail, and I second everything he (?) has said. I have a much more detailed piece in the works about my vision (well, ideas at least -- perhaps shouldn't call it a "vision" and overhype it!) for Boston's LRT, and a lot of it builds on the examples Brattle Loop gave.
For example, I have a crayon map I'm working on that builds out a full Huntington LRT subway, and provides one-seat rides to the LMA stop from: Kenmore, Brookline Village, Riverside, Needham, Jackson Square, Back Bay, Nubian, South Station, Park Street, Somerville, Waltham, West Medford and points north.
You'll never get a heavy rail line able to connect to that many locations. But because light rail has a, well, lighter footprint, it's easier to build flexible systems. Moreover, the large majority of the routes feeding into that Huntington trunk themselves come from dedicated ROWs, so you'll see high reliability, and most of those routes would also be suitable for longer trains. Modern light rail vehicles are able to offer level boarding, and at that point, you need to ask yourself, what benefit will heavy rail provide?
Additionally, like Brattle Loop says, turning Huntington into a full trunkline subway will enable overall increases in the capacity of the Green Line/LRT system.
Re Needham: @Brattle Loop, I'm not sure that what I have in store will cover that much new ground for you, I think we discussed it relatively recently. But I'm hoping to pull together something that balances being comprehensive and concise, have everything in one place.
Re automated light metro: yeah I agree, there's no hope until the Central Subway has a modern signal system. Automation also basically requires a "fully sealed" ROW, with basically no grade crossings -- I find it impossible to imagine the B, C or E being automated. That also, in my opinion, would preclude any subway that the surface lines run into -- my impression is that intermingling automated and human drivers is either infeasible or gives you the worst of both worlds.
If you built a full Huntington subway, connected it to Brookline Village at one end, and ran it along the commuter rail ROWs into South Station and then through the Piers Transitway -- I suppose that could form a fully sealed ROW, though a branch to Needham would put you on the rocks again.
If the T wants to invest in automation, I'd rather see the Red, Orange, or Blue Lines, where precise acceleration and deceleration can allow you to squeeze a few more trains into the subway. But even there, I think we're a long way off from it being worth it.