The $2.3-billion Green Line Extension is trundling towards completion, with the first station in the long-awaited project set to open this fall in Somerville.

GLX Constructors remains on track to complete the entire 4.7-mile extension between Cambridge and Somerville and Medford by the end of the year, at which point it will turn over the new tracks, trains and other equipment to the Massachusetts Bay Transportation Authority (MBTA).

Union Station in Somerville will become the first station on the GLX to go into service, followed by the Medford Branch station in December, according to a recent presentation by MBTA General Manager Steve Poftak.

Along with laying new tracks, GLX Constructors, a joint partnership between Fluor Enterprises Inc., The Middlesex Corp., Herzog Contracting Corp., and Balfour Beatty Infrastructure Inc., is also building six new stations as part of the project, in addition to the relocated and revamped Lechmere Station in East Cambridge.

STV Inc. is the lead designer on the design-build project.

GLX Constructors has kicked off the “rail activation planning process of really turning the GLX worksite into a functional railroad,” says John Dalton, project manager for GLX, in a project update. “We are getting close to seeing GLX become a real operating asset.”

News of the early opening of the two stations in the fall, and GLX’s continued progress towards final completion in late 2021, mark significant milestones in a long-delayed project that was on the edge of being indefinitely cancelled just a few years ago by state officials.

The Green Line Expansion has been in the works for the past three decades, part of a larger air-pollution mitigation deal between state officials and environmental activists in the wake of the Big Dig.

All 24 of the project’s light rail cars have been delivered, with plans to put trains on the newly-laid tracks and begin testing the equipment and infrastructure in the coming months, MBTA spokesperson Joe Pesaturo confirmed.

The Green Line extension is expected to draw 50,000 riders a day, helping to reduce congestion and air pollution on local highways and roads.

By the time the project is complete, the GLX project team will have laid down 100,000 railroad ties, 17 miles of track, nearly four miles of pilings, 22 million pounds of steel girders, while also rolling out two miles of local pathways.

“Construction on the Green Line Extension Project is more than 70% complete with revenue service targeted for the end of the year,” said a spokesperson for GLX constructors in an email.

In another milestone, project officials are also gearing up to turn over another part of the project - a new train maintenance and storage facility - to the MBTA as well.

“This facility represents one of the first assets GLX will be turning over to the MBTA,” Dalton told agency officials in February, during the last major project update.

State transportation officials agreed in 1990 to extend the Green line from a relocated Lechmere Station in East Cambridge, to Somerville’s Union Square and College Avenue in Medford, but plans for the project went nowhere for decades.

An initial attempt in 2012 to get the Green Line Extension finally moving ran into budget problems, and the most recent effort appeared headed off the rails for similar reasons in 2015.

Early cost estimates for the project ballooned from $2 billion to $3 billion, prompting then state Transportation Secretary Stephanie Pollack to order a pause and major overhaul of the project in 2015.

After consultants hired to review the project came up with $600 million in cuts, state officials opted to move forward with the long-delayed project once again. The cost-saving cuts included smaller stations and a smaller MBTA maintenance facility, a greater reliance on existing bridges, and a shorter bicycle and pedestrian path.

As part of the shakeup, state transportation officials also gave walking papers to the initial construction consortium on the project, opting to go with the Fluor-led GLX Constructors in 2017 after putting the work out to bid again.

Cambridge and Somerville also agreed to pony up $75 million to help defray the project’s costs.

So far, the project also appears to be on budget as well, though current estimates, which show spending closely tracking projections, don’t include change orders.

The project, when it opens, is expected to boost ridership on the T, especially in Somerville. Large sections of the city, which has undergone a real estate and development boom, are not currently serviced by subway or light rail service.

However, after the GLX comes on line, 80% of city residents will live within a half-mile from a T stop, up from 20% now.