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The MBTA is planning to open part of the Green Line Extension this October

Officials also revealed the branch assignments for the two GLX arms.

An MBTA map of the Green Line Extension at Ruggles Station in Boston. Courtesy of Edward Orde

MBTA officials have long said that they hoped the Green Line Extension into Somerville and Medford would be fully operational by December of 2021.

For some riders, service is slated to begin a couple months earlier.

Officials are planning to begin service on the GLX’s one-stop Union Square branch this October, before opening the six-stop Medford branch in December, according to a slide buried at the end of a March 29 presentation by MBTA General Manager Steve Poftak.

Asked about the schedule, MBTA spokesman Joe Pesaturo confirmed that “passenger service on the Union Square Branch is slated first, followed by the Medford Branch.” While officials have long planned to stagger the opening of the two branches in order to efficiently allocate staff for testing prior to beginning service, the suggested October opening date for the Union Square branch was not previously known.

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The schedule was first reported Thursday by StreetsBlog, after an MBTA rider posted a photo of an MBTA map including the Green Line Extension at the newly renovated Ruggles commuter rail station in Boston.

The map also indicates that the Union Square branch will be part of the Green Line’s D branch to Riverside Station in Newton and that the Medford branch will be on the E branch to Heath Street in Jamaica Plain.

Pesaturo noted that those two branches serve the Longwood Medical Area, and this will minimizing transfers for riders who take the commuter rail to North Station, as well as the Green Line Extension. When service begins on the Green Line Extension, the B and C Lines will terminate downtown at Government Center (currently, B trains terminate at Park Street and C trains terminate at North Station).

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The specific branch assignments are a reversal from earlier plans, which called for extending the D branch to Medford and the E branch to Union Square. Asked about the switch, Pesaturo said Friday that the D branch — which is by far the longest arm of the four Green Line branches — was assigned to the shorter extension to Union Square to make it a more reasonable length for trains.

“A shorter trip than the D branch, E branch trains have been assigned to the longer leg of the extension (Medford),” Pesaturo said in an email.

According to Pesaturo, residents will also begin to see Green Line trolleys running up and down the 4.7-mile extension “in the coming months” before it opens for riders, as officials test the new tracks. Some testing is already underway on the Union Branch and will commence this summer on the Medford Branch, Pesaturo said.

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Officials say the project, once completed, will increase the number of Somerville residents who live within a half-mile of a transit stop from 20 percent to 80 percent.

The project also includes a dedicated community path for pedestrians and cyclists, extending Somerville’s existing path — which connects the Minuteman Bikeway through Davis Square to Lowell Street — all the way to the Charles River in Cambridge. During a meeting in December, GLX Project Manager John Dalton said the extended path would open no sooner than the beginning of train service.

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