ONE FINAL DELAY of a couple of weeks looms, and then the long-awaited second and final Green Line extension branch will open to riders on December 12, MBTA General Manager Steve Poftak announced Thursday.

All five new stations along the new stretch will come online at the start of service that day, Poftak said, bringing rapid transit service to densely populated areas in Medford and Somerville where residents for decades have had to rely on buses or long trips to reach the Orange and Red Lines and downtown Boston.

The announcement puts an end date on the years-long, often rocky $2.3 billion project to lengthen the Green Line northward beyond its longtime terminus of Lechmere Station. Along the way, progress stalled or screeched to a halt on multiple occasions.

In August, when the MBTA temporarily shut down the Green Line extension’s already-opened Union Square branch for a combination of maintenance and construction needs, officials targeted late November to launch service on the larger Medford branch.

“We had some additional work we wanted to get done. We also wanted to be sure we were doing everything we needed to do, not only on the Medford branch but across our system,” Poftak said at an MBTA board meeting. “We’re taking a few extra days, but we will open at the beginning of service on December 12.”

Once the new branch opens, northbound Green Line service will split at Lechmere Station. Some trains will continue to Union Square, a stop that opened in March, while others will continue onward to new East Somerville, Gilman Square, Magoun Square, Ball Square and College Avenue stations, the last of which sits near Tufts University in Medford.

Trains have been conducting test runs along that stretch without passengers for several weeks, Poftak said.

MBTA officials previously projected that both branches on the 4.7-mile extension will boost the number of trips per day by about 45,000.

The extension’s roots date back to the Big Dig. In 1990, Massachusetts committed to extending the Green Line toward Medford in response to a lawsuit filed by the Conservation Law Foundation, which in ensuing years continued to press the state for transit improvements to mitigate environmental impacts of the highway project.

State officials originally targeted December 2014 to complete the Green Line extension, but a torrent of issues — including swelling cost projections and the switch to a new contractor — nearly upended it.

When it opened in March, the smaller Union Square Branch represented the first time the MBTA opened a new stretch of rapid transit service since 1987.

The Medford branch’s opening will serve as a sort of farewell for Poftak, who plans to step down as general manager on Jan. 3, and Gov. Charlie Baker, who will hand the corner office over to Gov.-elect Maura Healey on Jan. 5.

While both have faced sustained criticism for a bevy of MBTA problems and safety failures that prompted a federal investigation, many elected officials and transit advocates have praised the Baker administration for getting the Green Line extension across the finish line after its earlier uncertainty.

“I know there’s a universe of enthusiasts for this type of thing, so I look forward to seeing you all at approximately 4:45 a.m. for the grand opening of service,” Poftak said Thursday.