No matter how you cross into Canada, and return to the United States, you are responsible for having proper documentation with you.
If you choose to fly, you and your belongings will be inspected upon arrival at the airport in Montreal, Toronto, or Vancouver, and also upon departure from Canada to your home airport in the United States. If you drive, you, your vehicle, and your belongings are subject to search at the border.
AMTRAK's ADIRONDACK will be stopped at the border in both directions. Going towards Montreal, it will be inspected at the station at Cantic, Quebec by officers from the Canada Border Services Agency (CBSA). Going towards New York City, it will be inspected at Rouses Point, New York by US Customs & Border Protection (CBP) officers, a division of the Department of Homeland Security (DHS). You will be asked to provide documentation showing your nation of citizenship, and your date of birth. You may have your luggage searched as well for contraband.
AMTRAK's MAPLE LEAF will be stopped at the border in both directions. Going towards Toronto, it will be inspected at the station at Niagara Falls, Ontario. Going towards New York City, it will be inspected at the station in Niagara Falls, New York.
AMTRAK's CASCADES is not stopped at the border. All inspections in both directions are done at Vancouver's Pacific Central Station.
In addition, when you purchase your tickets for the above three international trains, you will be asked to state your nation of citizenship and your date of birth. If you buy your tickets via the internet, a special panel will ask you these questions. A questionaire also may be given to you by the train crew as the train approaches the international border. This information is then provided by AMTRAK to the personnel at the border when you travel in each direction.
AMTRAK's website provides a good description of what is required when crossing the border by rail from the United States into Canada, and from Canada into the United States.
The CBSA and CBP websites above should answer any questions you may have about what you can take across the border in either direction, such as medications, currency, food, etc.
Travel between the USA and Canada by air or water now requires a passport. Those crossing the border by land (car, train, etc.) will be required to have passports by January 13, 2008.