<P> The Green Line Extension (sometimes abbreviated as GLX) is an initiative to expand transit services in Greater Boston by extending the Massachusetts Bay Transportation Authority (MBTA) Green Line light rail beyond its current northern terminus at Lechmere in Cambridge, Massachusetts . The 4.3 - mile (6.9 km) extension is intended to improve mobility and regional access for residents in the densely populated municipalities of Somerville and Medford, two cities currently underserved by the MBTA relative to their population densities, commercial importance, and proximity to Boston . </P> <P> The project will extend service northward to Medford, near Tufts University, and to Union Square in Somerville using a two - branch operation, both to be operated within existing MBTA Commuter Rail rights - of - way . The extension is projected to have a total weekday ridership of about 52,000 . The Green Line would see an increase in boardings of 30,700, and the MBTA system would see between 7,000 and 8,000 new transit users . </P> <P> While on the drawing board since 1990, the project has proceeded in fits and starts . Construction finally broke ground on Phase 1 of the Green Line Extension in 2012, with service targeted to begin in December 2021 . </P> <P> The Green Line Extension is planned to have two branches, which will split just past a relocated Lechmere station in East Cambridge . The project's two branches consist of: </P>

When will the green line extension be finished
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