Driving, maybe not a lot, but probably more than we'd think. I've driven to and from Toronto area to Ottawa and Montreal a few times. My brother travels at least regularly between the area and Montreal, he currently flies. I think replacing flights and existing rail service is where you'll see a big uptake in passengers. A convenient and fast service can potentially attract more usage.
Some quick googling shows about 3k passengers flying between Toronto and Montreal daily. Another 3-4k passengers travel via train.
I like the idea of the HSR line, and I'd like more and better transit projects in Canada in general, I have very little faith in our ability to successfully execute such a project in a timely affordable manner. If the project ended up taking 25 years and costing $200+ billion to run only a handful of trains I wouldn't even be surprised.