U.S. citizens must have a passport for ALL travel (air, car, train) to and from Canada.
This is a summary of information available on the Department of Homeland Security’s website. Currently, all U.S. citizens must show proof of identity and proof of U.S. citizenship when entering the United States from Canada. Acceptable documents include: U.S. Passport Book, U.S. Passport Card, or other document approved by the Department of Homeland Security.
To determine which documents you require, contact the Canadian Embassy or consulate in your country. Following are links with additional information:Countries and territories whose citizens require visas in order to enter Canada as visitors