Can an unvaccinated Canadian citizen or noncitizen leave the country by car or train and likewise return by car or train?
The only train service between the U.S. and Canada is via Amtrak and its Canadian partner Viarail. Amtrak's website states (as of today):
In addition to the standard entry documentation requirements, the Canadian and United States Governments have several additional requirements for travel into Canada and the United States.
Travel into United States: Non-U.S. citizen passengers must be fully vaccinated with a U.S.FDA or a WHO approved COVID-19 vaccine. See details on the United States Department of Homeland Security website.
Travel into Canada: All passengers must use the ArriveCAN app before travel and wear a mask in Canadian stations and on board trains while traveling through Canada. Non-Canadian citizens must be fully vaccinated and carry the vaccination record that was uploaded into ArriveCAN. See details on the Government of Canada and Viarail websites, and additional Cantrail thruway bus service to Vancouver travel requirements.
The requirements are essentially the same for travel by car between the U.S. and Canada.