Perhaps cheaper than grade separation would just be flipping CN's yard around: There are 8 tracks in that yard, the closest 3 of which to the main tracks are also connected to the east. It appears there's enough land to connect the other 5 in the east as well, and with that done they could dead-end all the tracks in the west to eliminate the back-and-forth switching across Lancaster. No doubt CN would insist on being paid to do this by Metrolinx, and would heavily pad their numbers the same way they once soaked VIA Rail for the Kingston sub upgrades, but I bet it would still be a lot cheaper than grade separating Lancaster.
In theory that could create bottlenecks for GO's access to the Shirley layover yard, but not as much as you'd think. If you look closely, you'll see that GO can't directly access the west end of their yard from the south track without crossing over west of Lancaster anyways, so they're already regularly blocked out by CN's switching activities. Also, CN often parks cars for days on the north track under the River Rd pedestrian bridge, so I doubt that flipping their yard to east-only access would have much effect at all on GO, at least not today in a pre-AD2W world.
In theory that could create bottlenecks for GO's access to the Shirley layover yard, but not as much as you'd think. If you look closely, you'll see that GO can't directly access the west end of their yard from the south track without crossing over west of Lancaster anyways, so they're already regularly blocked out by CN's switching activities. Also, CN often parks cars for days on the north track under the River Rd pedestrian bridge, so I doubt that flipping their yard to east-only access would have much effect at all on GO, at least not today in a pre-AD2W world.
...K