All valid, especially considering Encore bought all those parcels east of Alford/Broadway with the intent of developing an "entertainment district" over time.
The one thing in the Eastern Route's favor is the ease of getting tracks there once you leave Sullivan Square. With the Northern Expressway viaduct looming over everything, that more or less forces you to cross that plane below-grade.
To take what's often the most popular form of Urban Ring station at Sullivan -- convert the freight sidings next to the OL platforms to Urban Ring tracks -- it might be doable if you assume smaller, automated rolling stock like Vancouver Skytrain, Montreal REM, which allow you tighter turning radii and steeper max grades than the ones
@Teban54 has catalogued for the Red and Orange Lines.
Using a 5% grade (
the lower end of the max range on the Skytrain), you can get the top of your tracks 32.5 feet up in the air from the trackbed elevation as it passes underneath the Mystic Ave bridge.
Without knowing the depth of a Skytrain viaduct off the top of my head (
they look pretty substantial, like 10-15 feet?) , I'd wonder if that's enough to keep from giving folks riding on Haverhill Line bi-levels a haircut, let alone leaving space for catenary. Remember that all those tracks start rising the minute you get out from under the Mystic Ave. bridge, too, so you're not gaining nearly as much headroom as that 32.5-foot figure suggests.
Curves would be doable using that smaller rolling stock, though:
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