A transit route feature was already a part of the Google Maps app, but only provided route information. The new Transit Navigation feature uses your Android phone’s GPS to track your location. With that information the app is able to tell you when it’s time to get off or transfer. The service will continue even when other apps are active and when you turn off your phone’s display and place it in your pocket. If you do this, your Android will alert you to route changes by vibrating.
Transit Navigation, like so many Google products, is considered a beta feature. Currently over 400 cities are supported, the majority of which are located in North America or Europe. More cities are will be added over time, although no specific timetable or list of targeted cities has been released. Since it relies on GPS, this feature will not work well in areas where a GPS signal is not readily available, such as most subways.
In addition to the new transit functionality, Google Maps 5.7 includes improved search suggestions and new support for viewing photos on Place pages.
Source: Official Google Blog