They'd need to do something about the hour-long stop at the border for CBP and its Canadian equivalent to go through the train checking everyone's passports. Decades ago I ran into it on the now-defunct Chicago-Toronto line and I'm pretty sure it's why they don't run those trains anymore. The New York-Toronto and New York-Montreal trains still do it.
Seattle-Vancouver avoids it by not making any stops between Vancouver and the border, so the immigration checks take place at the station. This might be feasible for Montreal, probably not for Toronto, and a train that runs from Buffalo to Detroit without stopping in Canada at all seems implausible.
Could probably set it up to do checks on departure. End up in the wrong country without your passport? Just take the next train back to the last destination in the other country.
Would likely need a special treaty in place so Americans traveling from Chicago to NY can travel through without a passport (just ID). Alternatively if we’re talking diplomatic solutions, the US and Canada could move towards a Schengen-style free transit zone without cross country border checks.
> Could probably set it up to do checks on departure
There's a seaplane from Victoria-Seattle. It's been a few years since I took it but I believe this is what happened. There's a custom agent at each side. I can't remember if there were any checks before departure though. I would imagine they would do some preliminary check because they don't want to be on the hook for taking you back.
Shortest custom wait ever BTW since the plane only holds 10-15 people.
> US and Canada could move towards a Schengen-style free transit zone without cross country border checks
This would be a dream. I'm curious why I've never really heard any proposal about this. As a Canadian (currently living in the US), I think that Canada would be more opposed to this. We always seem to have a fear of the US amalgamating us. I think it'd be politically tricky on both sides though. Even though it was proven to be false, there's still this myth that the 9/11 hijackers entered the US through Canada.
> As a Canadian (currently living in the US), I think that Canada would be more opposed to this. We always seem to have a fear of the US amalgamating us.
If it helps, Switzerland joined the Schengen area while maintaining its own customs controls (with reasonably consistent enforcement) and autonomy on immigration policy (outside of temporary tourist travel which is mostly harmonized). Major policy unification isn't necessary, although the minimum feasible level is likely still unprecedented for the US and Canada.
Seattle-Vancouver avoids it by not making any stops between Vancouver and the border, so the immigration checks take place at the station. This might be feasible for Montreal, probably not for Toronto, and a train that runs from Buffalo to Detroit without stopping in Canada at all seems implausible.