From the Washington Metropolitan Area Transit Authority's press release:
"Google's promotion of public transit increases awareness of alternatives to driving and gives our riders access to other useful information such as aerial imagery, business listings, user reviews and street views," said Metro General Manager and Chief Executive Officer Richard Sarles.
"In addition to making our transit data available on Google Maps, Metro is committed to making its transit data available so customers get the benefit of the best app tools that are developed for public use," Sarles said.
In August 2010, Metro offered its transit data to the general public for use in developing third-party applications for the Internet and mobile devices. Metro has been a leader in the transit industry in making public real-time data sets. Metro's transit data is available for download from Metro's web site."
Here is a list of the transit systems that Google Transit has linked to so far.
Thanks to Jeffrey Riecke