Earlier this year Google added a new transit layer to its own mapping application to show transit routes in the 400+ cities where it has data. Now they've announced this is available for the New York City transit system including subway maps. The result is an easy way to see nearby bus and train stops on the web or on some mobile phones.
Unfortunately, neither the data nor the imagery is available in the API version of Google Maps. While Google provides programmatic access to driving directions, as well as walking directions -- the third type, transit, has never been a part of the Maps API. The reason could be that there are extra factors, like fares and zones, that don't quite fit the same structure as other directions.
Indeed, though Google receives hundreds of transit feeds from hundreds of agencies, only a handful have data accessible to other developers. This may not be Google's fault, as I wrote in Why Aren't There More Transit APIs? It's easy to see the benefit of providing data to Google, but a harder sell to open up to any developers. Of course, some are better than others.
Developers don't necessarily need Google or agencies to provide them with all the data. London Tube Journey Planner, for example, shows routes and plans trips. We currently list 91 transit mashups, several dating back to 2005.