That's crazy. Perhaps a good comparison is the channel tunnel, e.g. going from London to Paris you go through security similar (a bit less onerous) to that at an airport before boarding.

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.)

Vancouver’s station does that: preclearance and then sit in a sequestered cage to get on the train.

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.

What about that piece of the US that's isolated from the rest, quite close to Vancouver I think, Fort something attached to the South of BC, accessible only through it. Do you have to go through something like that twice, or can you go between it and the main body of the US more easily (without stops perhaps)?

Or (facepalm, more obviously) Alaska for that matter?

Point Roberts is probably what you’re thinking of.

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.

The Adirondack train is suspended since 2020 and had a commercial speed of 56 km/h (35 mph).