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.

“Both boards now have a very clear sense of the path forward, if they want to take the path,” state Transportation Secretary Stephanie Pollack told reporters. “That said, there are substantial risks that remain.”

The new proposal, presented on May 9 by MBTA consultant Jack Wright, saves $288 million by building less-elaborate stations. The new, open-air stations feature weather shelters, instead of the originally designed canopy-covered platforms. There are also stairs, instead of escalators, and fewer elevators in the new design. The plan introduces at-grade track crossings that were not in the first design, but Wright stated they are typical for light-rail stations. The redesign also saves $122 million by including a shorter walking and biking path, which runs alongside the stations.

“The team was determined to stay conservative in our estimates to make sure we were not overstating anything,” Wright said. “We were definitely looking to build confidence. We recognize that was needed after what the project has been through.”

Officials conceived the project in the 1990s to mitigate the environmental impacts of the Big Dig. The project swelled $1 billion over its $2-billion budget, causing state officials last year to put it on hold. The plan was nearly abandoned when it came to light that the MBTA had based its plans on botched budgets that, among other things, misjudged rising labor costs. Station designs got more complex and expensive as the project progressed.

The MBTA initially attempted a contracting process that it had never used before. The construction manager-general contractor method allowed the MBTA to sequence the work in packages as the designs were developed, with input from the contractor. But after that process went south, the agency in December ended its contracts with four consortiums: general contractor White-Skanska-Kiewit, project manager HDR-Gilbane, estimator Stanton Constructability Services and designer AECOM-HNTB.

Now, officials plan to use design-build to bid the project as a single contract. The MBTA has limited experience with the project-delivery method and, to date, none of its design-build projects are as large as the Green Line Extension. The MBTA needs to hire a new project management team to “own” the design-build procurement process, Pollack added.

Speaking of the design-build approach, Pollack said, “That’s a great thing. We just need to be very careful not to make the same mistake as before and make sure that we’ve done enough of the organizational and capacity-building. [We need to make sure the] new team … is in place before we hit critical procurement milestones that we are going to want to hold them accountable for.”

While the project is estimated to create 41,000 construction jobs and $1 billion in development, Pollack worries that there won’t be enough revenue to complete it as designed. She also is concerned that the Green Line Extension will cannibalize ongoing efforts to maintain existing infrastructure.