In the Phoenix area, supported vehicles include Valley Metro buses and trains, and the PHX Sky Train at Phoenix Sky Harbor International Airport. Routes extend to and from suburbs such as Chandler, Mesa, Glendale, Scottsdale, and Tempe.
In the Birmingham area, supported vehicles include MAX Transit buses operated by the Birmingham Jefferson County Transit Authority. Transit directions have also recently been enabled in Montgomery and Huntsville.
MacRumors reader Bernd Keuning, who keeps close tabs on Apple Maps, also sees hints of transit directions eventually expanding to Nordic and Baltic countries. Progress is visible in Denmark, Sweden, Norway, Finland, Estonia, Latvia, and Lithuania.
Keuning has a good track record, having tipped us ahead of time about transit in several cities and countries where the functionality eventually launched, including Las Vegas, Phoenix, Ottawa, Paris, Rome, Madrid, Taiwan, and Singapore.
The telltale sign is that Apple Maps typically adds station outlines to a region shortly before launching transit directions in the area.
Apple Maps gained a Transit tab in iOS 9. The feature lags several years behind Google Maps, but Apple's public transportation support is at least exhaustive, mapping all station entrances and listing departure times. Apple Maps also provides detailed advisories about service interruptions and other changes.
At launch, the feature was limited to Baltimore, Berlin, Boston, Chicago, London, Los Angeles, Mexico City, New York, Philadelphia, San Francisco, Sydney, Toronto, and over 300 cities in China. Since then, Apple has been working to expand support for public transportation to other cities around the world.
For an occasionally updated list of cities and regions with Apple Maps transit directions, visit the iOS Feature Availability page on Apple's website.