Its pretty much hard to get there without a lot of work. You would have to extend the Union Sq. branch of the Green Line extension all the way out to Alewife, which would require getting past Porter Sq., among other obstacles where the ROW isn't wide enough. Then, you'd also have to deal with the fact that the Watertown Branch is a single-track ROW and the grade crossing of Fresh Pond Parkway. Finally, you'd have to account for the fact that it would probably be cheaper to upgrade the 71/73 trackless trolleys to BRT standards than spend all that money getting the Green Line extended for that circuitous a route.
Wondering if I'll see the Western Route double-tracking finished before I retire...
Photo: Melbourne W7 No. 1019 on Route 78, Bridge & Church Streets, Richmond, Victoria. 10/21/2010