King County Metro (Seattle, WA) provides free WiFi service on its buses. Photo by Daniele Muscetta.
The East Bay AC Transit is testing wireless internet equipment and plans to offer free WiFi on its Transbay bus routes by the end of the year. Only a handful of other public bus systems in the US provide WiFi, including transit operators in Seattle, Riverside, Cincinnati, Tampa and Cedar Rapids.
In the San Francisco Bay Area, the ACE Train between Stockton and San Jose and the Amtrak Capital Corridor that runs between Sacramento and San Jose offers free WiFi service to its passengers. Caltrain recently completed proof of concept testing and is looking for funding to implement free WiFi over the next year or two.
BART has no concrete plans for WiFi -- they're still trying to get cellphone service in the Transbay Tube.