Funding source? City of Boston & TIF in East Boston.
Personally, I think this is kinda like an "infill" station that the locality (Boston, in this case) should pay for and quit all the bellyaching about how the state has done 'em wrong. Yes, the State jerked the city around on this, but Boston is absurd/pitiful/stupid in its state of
Learned Helplessness on this, for which I blame the Big Dig (Boston got in the habit of other people picking up the check for key core infrastructure, and the Mayor throwing around what should have been TIF funds on non-transport)
The City of Washington DC paid for its
NoMA station and the City of Chicago is
paying $50m for an infill L station at McCormick Place using TIF.
Where are all these Blue Line riders whose lives will be transformed? Boston. (Maverick, Airport, Wood Island, Orient Heights, Suffolk Downs, and half of Orient Heights). And that's where all the TIF should come from, since that's a whole lotta Boston that'll go up in value.
Did Somerville pay for its infill at Assembly? No. So what do we make of that? I say Somerville won the lottery. A random act of stimulus, and there's no generalizable conclusion except that Boston, if it wants Red-Blue should save its pennies, work hard, and build it itself. Instead, they keep buying scratch tickets and booze with the money that coulda had this thing built.
Would it be cheaper to wait for the state? Sure. Everyone prefers to wait for some other guy to shower infrastructure on 'em. But if it is so rip-roaring beneficial to 5.5 Boston neighborhoods, (6.5 if you count Beacon Hill / Bowdoin / MGH) Boston should man up and pay for it and quit asking for it for Christmas.
The "next Seaport" should be the Maverick/East Boston Waterfront. TIF revenues there should go to toward paying for Red-Blue. Heck, it could be the next biotech hub if connections to Cambridge are easy.