According to the VIA FAST proposal (available on
http://highspeedrail.ca/), there are 4.4 million trips between Toronto and Montreal annually, 50% by car, 33% by air, 14% by train and 3% by bus. Increasing departures to 10 a day and increasing speed to 200 kmh (3 hours 30 minutes travel time), they expected to get 19% of the market, mostly taken from air (a 35% increase).
The report states that where true high speed rail has been put in, traffic volume increased by between 2 to 7 times. 7 times seems unlikely between Toronto and Montreal (as that would be all the traffic). I think a true high speed train could capture 60% of the air traffic (870k annual trips) and 20% of the vehicle traffic (440k annual trips) and 100% of the train traffic (obviously - 600k annual trips). That 1.9 million trips, an increase of over 3 times. I'd expect fares to be similar to air travel on a full service air carrier.