Apple has announced Transit supports in Maps at the launch of iOS 9. Since then Apple is continuously improving it and bringing it to new cities around the world.
Now this time it brings support for Denver, Colorado. Which means that iPhone users can use Transit service in this following areas.
Atlanta, GA
Austin, TX
Baltimore, MD
Berlin, Germany
Boston, MA
Chicago, IL
China
Denver, CO
London, England
Los Angeles, CA
Mexico City, Mexico
Miami, FL
Montreal, Canada
New York City, NY
Philadelphia, PA
Portland, OR
Rio de Janiero, Brazil
Sacramento, CA
San Francisco, CA
Seattle, WA
Sydney, Australia
Toronto, Canada
Washington, DC