Northfield / 85 Exchange upgrades?

[Canard]

The point is to take advantage of the fact that there is absolutely no traffic crossing the space between the two tracks for a long distance to put non-vehicular traffic in that space.

I think you're making an assumption that there is a big wasted space between the tracks.  In fact, there is not - what little space there is between the two tracks is occupied by the poles which support the Overhead Contact System ("Catpoles"):

   
I've taken the liberty of ghosting in the approximate dimensions to scale of the LRV's, at 2.65 m width.