Mark Gurman, writing on 9 to 5 Mac:

Apple currently plans to debut bus, subway, and train route navigation as the central upgrade to the Maps app in iOS 9 at WWDC, using a user interface similar to the one intended for last fall’s launch […] Due to personnel issues, data inconsistencies, and coverage for only a small subset of Apple’s major markets, Apple decided to pull transit functionality from iOS 8 very late into development, sources indicate. The feature was apparently present in internal iOS 8 betas well into the summer of 2014, despite the lack of an announcement at last year’s Worldwide Developers Conference. Since that time, however, Apple has refined the data, added new cities, and developed a new push notifications system that will notify users as new cities gain support.

I don’t have any use for transit directions. But for many, this is a necessary feature that will be a welcome addition to Apple Maps.