The Montreal-Quebec trip is definitely an example of what can cause problems. Those who are familiar with the area know that trains from Quebec City have to go all the way to Dorval and turn around before coming back to the downtown area; trains cannot easily access the downtown area.
I think we have to ask ourselves the following question. Does the high-frequency train really need to get into downtown Montreal? There is actually a mantra according to which the train must go to downtown Montreal. However, if the answer to my question is no, it is possible to reduce the travel time by 20 minutes. For example, there could be a station in the Saint-Laurent borough and a mega-station in Dorval. People could also use the Réseau express métropolitain, REM, to get to Saint-Laurent in four minutes.
In Japan, a number of high-speed train stations have been built in the suburbs, including in Osaka and Tokyo, because bringing them downtown would require tunnels that are too expensive. So there is a way to optimize the route and the journey, even though I know that not everyone agrees on that. Some people believe that trains should go to downtown Montreal. But if I can reduce the trip time by 20 minutes, including the time spent on the REM, I would personally prefer that option.
For the rest of the line, the time savings would be better, including for the Ottawa-Montreal trip. You have to remember that the time VIA Rail is proposing for the trip between Ottawa and Montreal with a high-frequency train is almost identical to what it was more than 20 years ago, one hour and 21 minutes.