Upon arriving at the train station and registering your passport and documents will be verified. Your documents will also be verified when you arrive in Vancouver.
The Amtrak website provides some guidance for this:
The form of identification that you carry on your trip must be the same identification you selected and provided details on when you made your reservation.
You must carry the valid original identification document. Copies, expired or invalid documents will not be accepted. Each passenger must have their own identification.
It is recommended to arrive 1 hour before departure.
Amtrak waterfalls from Seattle to Vancouver: how is the border crossing north? Summarize this very well:
As a reference, this train performs customs clearance from Canada upon arrival at the Central Pacific Station in Vancouver, not at the border.
There will be a check at the station in Vancouver and another customs check when the train arrives in the United States.
It is also recommended to arrive 1 hour before departure.