That's a lot of qualifications in my headline: The Washington State Ferry system has the very largest Wi-Fi operation for regular transportation in the United States by far--they carry 50 percent of the passenger trips in the US across their boats--but it's rather hard to find No. 2, because there aren't many production Wi-Fi services in effect. There are trials here and a few buses or train cars there. ACE Transit in the Bay Area thus clearly becomes the second largest commuter-Fi and largest land-based-Fi service today.
Internet access is available on specially marked buses--78 MCI buses and a dozen others--that cross the Bay Bridge, San Mateo Bridge, and Dumbarton Bridge. The system has 11,300 daily riders. The service is free, with the capital expense funded by a county grant. AC Transit's monthly cost, they say, is just $60 per month--the cost of the cellular data backhaul. They say adding just one new commuter per bus per month would cover that cost. (The press release isn't posted at this writing.)