There is definitely not room for a 4-track ROW (even a 3-track ROW) continuously along the Fitchburg. Any Red Line extension would require significant tunneling in places where the population density, while warranting some sort of rail transit, wouldn't warrant the cost of subway construction.
And, it would cost an unholy fortune -- it would never survive planning review due to excessive capital cost per new passenger generated.
IF (and, as stated, it can't), but if the ROW were wide enough to share rapid transit and commuter rail tracks (in their seperate areas), then the most logical choice anyway would involve a phased extension of the Green Line past the planned Union Sq. (Somerville) terminus, if the "Beyond Lechmere" project ever takes off, but that runs into the above bottlenecks (Belmont center, Waverly underpasses, the cut in Waltham, etc.) but also a blocked path at Porter Square. But, if all of those weren't in the way, a Green Line extension would be cheaper, more in scale with the communities, and also wouldn't require the closure of the grade crossings (seven in all I believe between Union Sq. and Brandeis/Roberts) as well.
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