@hebrewhammer -
The word is that the “last” building is not contingent on the on the Green Line extension.
Apparently, the concept for such a building dates back to the previous century - predating the Green Line extension project.
The key dependency has been securing “air rights” over the train tracks, which is easier in the current economic climate than in the past.
The MBTA is actually seeking partnerships like the one with Tufts to help defer the costs of building the extension.
It will be interesting to see how the Green Line extension project plays out, as it is a number of years behind schedule.
The hope is that through a combination of partnerships and reductions in scope/cost it will proceed - as it would bring a number of benefits to Somerville/Medford/Tufts above and beyond the building itself.
The Red Line extension in the '80s, coupled with the elimination of rent control in Cambridge in the 90’s brought major changes to Somerville/Tufts and particularly Davis Square.