^ I think part of the issue is
where the Wilson Yard is located vs the Richmond Hill Layover in the Don Valley. Wilson's on the south side of the existing tracks and Richmond Hill is on the north side. If you look through the material for the Union Station Rail Corridor East Project, the location difference matters. They want to reduce crossover movements and have certain needs for cycling certain lines. See slide 13
here from
this site.
In other words, the Richmond Hill Layover being located on the north side may strategically benefit the Richmond Hill and Barrie Lines since they branch out from the north sides.
The Richmond Hill Layover also might be needed now because
they aren't going to extend track E0 after hearing concerns from local residents who live along the track. This is speculation on my part.
@smallspy or
@crs1026 may have a view.