As of April 15. 2020, the government of Canada is now requiring people who enter Canada to wear a non-medical face mask to prevent the spread of COVID-19. A mask will be provided if you do not have one.
The government will be conducting spot checks. There are stiff fines for those who fail to comply with the order.
The press release may be found here: https://www.canada.ca/en/public-health/news/2020/04/government-of-canada-updates-mandatory-requirements-for-travellers-entering-canada.html
Guidance for transit operators and the use of face masks is attached below.