Amtrak provides Wi-Fi service on the Northeast Acela run, and many regional train operations also offer Internet access. Over 800,000 passengers rode part or all of the Cascades line during 2010, the high-water mark for ridership. I have a trip planned to Portland in February, and might opt for train over car because of the Wi-Fi.
Service is free for now, and may remain free; it depends on usage and other factors. Internet service is typically installed to provide operational service for the train line, as well as passenger access, but it's unclear on whether this is just an amenity.
Washington, DC, Puts Free Wi-Fi on Mall