I do wish people would give up on this. Unless both are designed concurrently, grade and curve requirements are totally different.
In this case, there is no way to feasibly run rail up Hwy 10, even if the space existed in the ROW. Gaining altitude up the moraine, roads can do it much more abruptly. That's why rail lines of full of reverse curves, horseshoe curves, etc. unless a lot of bridging is employed.
At the least, there’d have to be a very long viaduct or earth fill between Sligo Hill and the Escarpment itself. Really, the fastest route between Orangeville and Toronto is via the old TG&B route through Bolton, though it’d need some major rerouting in sections to get around Caledon Village and the Horseshoe Curve.