I haven't taken that train in many years, but they basically stop the train at the border and immigration agents board and check everyone's passport. It's scheduled to take 2 hours. Really stupid. It's a 45 minute flight, and you go through US immigration in Canada before boarding the flight.
That was the case even with the UK in the EU (maybe it's not any less onerous than an airport now actually, idk) but otherwise intra-EU over land is not an issue, almost necessarily. (But then, you might think that about Canada/US.)
https://en.m.wikipedia.org/wiki/Pacific_Central_Station
There are some disadvantages, such as not being able to pickup anymore passengers until crossing the border. Probably a non-issue with Vancouver close to the border.
Or (facepalm, more obviously) Alaska for that matter?
Popular with Canadians to send parcels to and pickup gas.
https://en.m.wikipedia.org/wiki/Point_Roberts,_Washington
A school bus runs from there to mainland USA and I suspect there’s an informal agreement to drive without stopping through Canada so they don’t have to bother too excessively with clearances.