Online mapping pioneer MapQuest is using open data to provide transit directions via its MapQuest Directions API and the OpenStreetMap-based MapQuest Open Directions API. In both cases, transit directions are based on the Google-created GTFS data standard that helps transit agencies share their routes, schedules and fares in a consistent format. Currently MapQuest's support is limited to six U.S. metro areas.
The MapQuest developer blog has the transit directions technical details, including a sample request. You can also see the advanced routing example and choose the multimodal route type to create a query using a form.
To start, the API supports transit directions in New York, Chicago, Washington, D.C., Boston, Philadelphia and San Francisco. MapQuest's Kumiko Yamazaki said the initial release was constrained by quality control. "MapQuest's transit option covers six major metropolitan areas, reaching roughly 90% of our nation's rail ridership," Yamazaki said.
We have heavily covered open transit data in the past, including an interview with Jehiah Czebotar, a developer evangelizing open transit. Czebotar's aim is to get transit agencies to support the GTFS standard, the same one MapQuest was able to use to add directions to its APIs.
GTFS, which originally stood for Google Transit Feed Specification when we covered it in 2008 (the G now stands for General), was created by Google in coordination with Portland, Oregon's TriMet. Now there are over 100 transit agencies with public feeds. With MapQuest's first set of metro areas completed based on the standard format, one could expect to see more cities added to the transit directions service in the future.