We already have a rail line across the country that works just fine for freight which doesn't care as much about travel time. The only added benefit to a high speed rail line across the country would be faster travel of humans which is currently handled by road based vehicles or airplanes, there's no significant value addition in a high speed rail line that would make up for the astronomical cost over such a distance.

Unless you plan on turning Canada into a 300 million person country, the size to population ratio simply doesn't provide for a use case for "connecting our own country". We barely use any of our land for people right now, even along the US border where we are mostly tightly grouped. Half our population is in Southern Ontario/Southern Quebec, with only a half dozen other major cities worth mentioning across the entire country.

Economies of scale matter, and Canada simply doesn't have enough people to reach those for transportation networks across the country.