Much of that funding would come from the federal government, according to the plan.
The plan says $1.34 billion "will fund procurement of vehicles, construction of stations and improvements to rail and signal systems to enable service to Somerville and Medford by FY2020. The [Capital Investment Plan] assumes that the Federal Transit Administration will fund almost half of the total cost of the project, which would reduce the required state match to $730 million through FY2020."
Although this doesn't present a new reality for the Green Line Extension, it's worth noting its inclusion in the transportation plan.
Construction on part of the Green Line Extension—to Union Square and Washington Street in Somerville—is already under way with $393 million in state funding.