Fun fact: the most active air-route in North America is Toronto to NYC, which is about a 750km drive if you try to do a direct route, 850km if you follow the current Amtrak route through Albany, which hits all the major upstate cities for you.

Dedicated high speed rail goes about 300-350km/h. It would be reasonable to image that trip taking 2.5 hours, maybe 3.5 or 4 hours if you do the Albany route and milk-run all the stops like Hamilton, Niagara, Buffalo, Rochester, Syracuse, Utica, and Albany.

The current rolling stock can hit 200kph; so proper tracks alone can bring that voyage to 2h45.

A flight is 1h20 each way (average) but requires you to be at the terminal 1h in advance. We're taking a 20 difference between flying and the train. Of you need to be downtown, that time is easily saved by Kot needing the UP express or (soon) the REM.

Unfortunately operating speeds are not start to stop. That would probably still be around 3h15. The TGV can do ~270, getting us to the two hours.

HSR plus a France style short-haul flight ban would flights everything inside Toronto-Ottawa-Montreal and drastically reduce Quebec and the horseshoe.