You’ve heard of the Red Line, the Orange Line, the Blue and the Silver, even the Green Line. But do you know about the GLX? That’s local transit lingo for the Green Line Extension, the long-delayed plan to stretch the Green Line route into Somerville and Medford. The GLX, envisioned for decades, has been plagued by problems. Last year the state revealed it could be $1 billion over budget; officials halted the project, canceled contracts, and rethought the details of the plan. Under the latest proposal, the seven new GLX stations are scheduled to open in 2021.