The Bay Area Rapid Transit (BART) system that links up much of the San Francisco Bay Area is in the first phases of a Wi-Fi rollout: As reported on this site earlier, through intelligence from veteran Wi-Fi guru Matt Peterson, Wi-Fi Rail has been turning on Wi-Fi at BART stations without any publicity. The company says that 1,000 riders have used the Wi-Fi service so far, with more each day (look for a spike today, no?).
The current test involves four stations in San Francisco (Embarcadero, Montgomery, Powell, Civic Center). The test could lead to Wi-Fi Rail getting a contract to install service through the system's stations. They will likely charge $10 per day, $30 per month, or $300 per year.
While those interviewed in this article aren't jumping up and down about Wi-Fi, that's probably because of the lack of continuity of service at this moment. Most transportation-based Internet access--ferry, plane, or bus--offers service that starts and ends at stations or stops, but is available throughout. That's a long-term goal for this installation, according to BART. When people realize they can stay connected on a phone call, or use a handheld device with Wi-Fi throughout a trip, they'll be a bit more enthusiastic than they are about whipping out a laptop at a station.
While Wi-Fi is an option on several long-haul/cross-country trains in Canada, the UK, and Sweden, adoption in commuter rail, light rail, and subway systems will likely take quite a bit more testing before becoming commonplace. The challenges of routing signals across complex routes, much of which might involve underground tunnels, adds a lot of cost and complexity in planning, installing, maintaining, and troubleshooting.