The Green Line Extension's Medford Branch will open for service on Dec. 12, MBTA General Manager Steve Poftak announced during a board meeting Thursday.
The Medford branch of the extension project was previously slated to open sometime in November. Test trains have been running on the branch, and many locals are awaiting the official opening, which is now set for next month.
"We had some additional work we wanted to get done," Poftak said. "We also wanted to be sure that we were doing everything we needed to do not only on the Medford Branch but across our system. We wanted to make sure we did it safely and did it properly."
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The MBTA has been working to finish training trolley operators as well as safety testing.
Five additional stations will be opened along the Medford Branch. 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.
Over the summer, the MBTA announced a delay in the project in the wake of the Federal Transit Administration's safety report.
This latest phase of the Green Line Extension will use existing Lowell Commuter Rail tracks to take riders from a relocated Lechmere station to Medford.
The Union Square branch of the Green Line Extension opened back in March.
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.
Once the Medford extension is complete, E Branch trains will terminate at Medford/Tufts, while D Branch trains will terminate at Union Square.
