Exactly this. The CP North Toronto Sub. (and later the Agincourt yard) and CN York Sub. were built in farmer's fields. Both are the mainline routes for both companies. If some kind of shared by-pass corridor is proposed, both will want its capacity to be sufficient to meet both of their unconstrained traffic and be reasonably future-proofed. Any purchase price (or consideration) would have to be favourable - they are both for-profit companies. And, of course, a willing host municipality.
Whenever people see a train of sea containers go by, they have to remember that a lot of the contents are consumer goods and destined for the GTA. The farther the terminals are pushed out, truck traffic will increase.