I don't know how many trains run on the Galt sub today, but a few years ago it was around 7 or 8 - really long ones, to be sure, but once they were out of the way, they're out. If the number remains similar then I can't see how much of a constraint it really could be to have passenger trains running every 30-60 minutes on it.
That approach has its own set of complications - there aren't many locations along the corridor where there's room to add 2 more tracks without running afoul of the various industrial lots. The Humber River crossing would also need an entirely new bridge constructed. And I think there was also a discussion somewhere of how, if ML were compelled to build their own tracks, they would have to rebuild all the stations to sit on the south side of the corridor, though I'm not entirely sure as to why. Taking all of that into account, I'm not sure how doable it actually is.
The easiest solution would be to declare a provincial emergency, and evacuate Milton as a failed town.