Not just Montreal to Toronto. If you've ever been to a country with HSR here's what you'll see:
1) It really competes with air. Right now. If you're going from downtown Toronto to downtown Montreal, door to door including pre-boarding, access times to the airport, etc, it's easily 3-4 hrs. HSR would be 3 hrs, cheaper than air and more comfortable and productive.
2) Supercommuters. It's very normal to see folks commute 150 km with HSR. Especially white collar. Being a lawyer or accountant or doctor in Toronto and being able to live in Peterborough would be huge. Being able to live in Ottawa and access the Montreal job market would be nice. Etc.
People who have never experienced HSR will look at this at something like VIA but faster. But if you've ever traveled, you'll see HSR stations function more like a mini airport terminal with giant parking structures, car rental counters, transit connections, shopping arcades, business centres, etc.
Finally, it should be noted that Canada is the only G7 country without HSR. And population density in the Quebec-Windsor Corridor is actually close to European HSR corridors.