So, in this configuration, would the Green Line run at grade next to the Fitchburg Line in a four track right of way from Union Square to Beacon Street, where it would then enter a tunnel between Beacon Street and the Porter Square station under the commuter rail tracks? It seems as though a lot of bridge work and building demolitions would be required along the at-grade 4 track right of way between Union Square and Beacon Street, given the zero-setback footprints of many of the old buildings. Is this in fact the case, or is there enough clearance to run four tracks at grade up to Beacon Street?The stilts of the building are 4 tracks wide, because it bowlegs over the ancestral ROW which was quad all the way through Belmont Center. The tunnel footprint would only be somewhat wider than the island platform here, with staging not needing to touch the building stilts. The only question is whether you do some manner of delicate construction staging so 1 temp track of the Fitchburg can stay in-service through the duration, or sever the whole works for a year with "rip the band-aid" approach and alternate shuttle routes for the outbound stops.
Some additional space on the extension gets saved by the Green Line's electrical being tied into the existing Red Line/TT substation in that overhang building (the vent windows seen on the photo), meaning you can anchor the power draw for the extension with distributed power upgrades to the Porter + Alewife RL sub power rooms and the next-nearest GLX sub rather than needing to construct an all-new substation. That's a significant cost saver for this particular extension being able to span existing 600V interconnects rather than create all-new substation sources like the current GLX has had to.
It's also potentially feasible to have the tunnel construction intentionally lower the level of the Fitchburg trackbed a couple feet before reconstructing it on the roof, making the Porter-Beacon St. canyon here able to be capped over at sidewalk level for an air rights park (and, crucially, a direct Somerville Ave. driveway for Lesley U./Porter Exchange so inadequate Roseland St. no longer has to be tasked with acting like a thoroughfare between Beacon & Mass).