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I did this exact route from YVR to SEA on Amtrak 10 years ago.
Honestly, it is pretty slow.
I think took 5 hours as there is only 1 track.
Quite a few times we went forward, and then the train had to back up to let another train pass.
Also US Customs boards the train and that takes a while.
Maybe if there is a bus, I would do that.
Probably faster.