This certainly wasn't the case when I worked for Fedex at YYZ. Unless a package was going to a very remote area, it was likely to move by truck once it arrived in Toronto. As a matter of fact, anything in Southern Ontario and Quebec moved by truck. If you ship something from Toronto to Ottawa or Montreal via Fedex, it never leaves the ground. I think the exceptions are where the Caravans operate which is northern Ontario if I remember correctly.
I've worked for both UPS and Fedex in my younger days. In fact, I was at UPS and sorted the first inbound air packages they had into Canada in 1987. There were two of them (yup just two small packages for their Next Day Air offering) and they came into YHM on an old turbo prop metroliner. It still amazes me to this day to see them operate the MD11into YYZ after that first flight in June of 1987 with just two packages!
Hamilton wasn't the best for Cargo Ops - at least for UPS. Once space became available at Vista, UPS jumped at the chance to move to YYZ. Fedex could have built at YHM in the late 90s but they didn't. They built that massive facility at YYZ. I'm sure that if they could, they would build at Dorval rather than Mirabel. It's got nothing to do with freight interlining to passenger aircraft but everything to do with being close to the metropolitan areas so freight can get to the sort faster.
Less and less freight moves in the belly of passenger jets nowadays for a variety of reasons. The dedicated freighter traffic that comes into YYZ now just blows my mind.
Unless your HSR is hourly, you're not going to see a large reduction in flights. Some flights are also positioning flights to move the airplane for international ops and maintenance visits.
The fascination some members have with high speed rail and a second airport for the GTA I'll just never understand.