My post was based on experience, not theory. The train stops at a special post that has minimal staff. The bus stops at the main Niagara border office at the Peace Bridge and they typically can expedite things there. As for the UK visitor, it is absolutely necessary for a visitor from the UK to have both the stamp and a card for exit. I saw it happen in person less than a year ago. My friend gave the US card in at a land border (to the Canada border agent) into Quebec upon return to Canada. As for the OP, flying makes a lot of sense. The train is very slow and not especially comfortable. The Amtrak line in NY is a money loser that makes a lot of stops. The OP has made the right decision to go by air but perhaps should have chosen Porter.