As BobH has said, you go through customs & immigration formalities on arrival in Vancouver. Last year, it took at least 40 minutes. Also, you need to allow for the fact this service does often operate late. Our southbound train was delayed due to i) other trains in the Lower Mainland (CNR, Rocky Mountainer); - and - ii) a fatal accident @ Bremerton (pedestrian on track) requiring a 2 h delay waiting for clearance to proceed. (NB I did get a full credit from Amtrak for the delays).

My point is do not take their schedule as given! IMO, take Bolt Bus and allow time for border formalities.