Massachusetts transit officials this month took an embattled plan off life support to extend the most iconic line on the nation’s oldest subway system. But the long-awaited Green Line Extension project still faces a lengthy revamp after it was halted last year—and it could still be nixed altogether.

The Massachusetts Dept. of Transportation board and the Massachusetts Bay Transit Authority’s fiscal control board voted to advance a scaled-down version of the 4.5-mile extension into Somerville and Medford. The $2.3-billion plan, which includes $700 million in sunk costs, is still short $73 million. The Federal Transit Administration, which pegged $1 billion for the original plan, would have to approve the new plan, too. In announcing the restart, state transit officials made it clear that they can pull the plug on the seven-station extension at any moment.