The first priority would be safety. It would be how the freight and passenger operations interact with one another, and as you mentioned, the number of grade crossings you would have to deal with, trespass issues, and so on.
The second priority would be schedule maintainability. Again, how do the freight and passenger trains interact? How do they delay one another? What sorts of impacts do you see? Can those be mitigated, and if so, how?
The first step would be a major risk assessment. Everyone--locomotive engineers, track maintenance people, the people who do the actual work--would be brought together to do a detailed risk assessment. Transport Canada rail safety people, as well, would be at that table to work through that in great detail.
The other priority, if I may--I should have said this--is that you really have to define what it is you are looking for. Is it 110 miles per hour? That is one thing. Is it 125 miles per hour? That is quite different. As for 150 miles per hour, I just wouldn't go there in terms of a comingled operation. I would strongly recommend against doing that.