BOSTON CBS) – The Green Line extension, a $2.3 billion project that expands service for 4.7 miles along two lines into Somerville and Medford, is now more than 50% complete. Gov. Charlie Baker called the project, which includes construction of six new stations, a “huge, game-changing project.”

Groundbreaking took place on the project in 2018 after discussions that stemmed back to the 1990s. The Green Line extension is scheduled to be completed by the end of 2021.

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“Before this project, 20 percent of the people of Somerville were within a quarter mile of a public transportation stop,” said Baker. “When this thing is finished, 80 percent of the people in Somerville will be within a quarter mile of a public transportation stop.”

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MBTA General Manager Steve Poftak said crews will have installed 100,000 railroad ties, four miles of pilings, 22 million pounds of steel girders, 17 miles of permanent track and two miles of a multi-use community path.

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“When it’s finally done at the end of 2021, the GLX (Green Line Extension) will deliver new rail service to transit dependent communities in Somerville, Medford, and Cambridge,” said Poftak.