The Green Line Extension is in the home stretch. Here’s an update on construction. “From a big picture standpoint, we are right now at the eighty percent complete point, which is a long way.” The MBTA Green line passengers with masks on a weekday morning during COVID-19 coronavirus pandemic. (David L Ryan/Globe Staff ) Email The Green Line Extension (GLX) into Somerville and Medford is 80% done, but there’s still a ways to go. At a Wednesday meeting, the GLX team shared updates on construction progress and confirmed some long-anticipated opening dates. The Union Square branch will open this December as part of the D line, and the five-stop Medford branch will open in May 2022 on the E line.