The Altamont Commuter Express (ACE) was the first train route to add Internet access: The service has been available in just one car along the fairly long commuter route that the train takes from San Jose to Stockton. In historical terms, the line runs from what was all orchards (when I grew up in Fremont, Calif.) to what was all pasture and farmland.
The operator is looking at a switch to a new provider which would allow them to boost speeds, almost certainly by switching to an EVDO network instead of the current slower system (a combination of satellite and cellular, the last time I spoke to the firm that operates ACE's current service), and to fill in some coverage holes.
I wrote about ACE in my planes, ferries, and trains article for the New York Times in summer 2004--the article covered Internet access that was ostensibly arriving for commuters across a variety of transportation methods. The growth in access has been steady, although municipal wireless may accelerate it enormously.