For this type of deployment, Wi-Fi may not be the best fit. Although Wi-Fi can provide very fast roaming times (we target 40-80ms handoffs) between APs, it is really designed for people walking and not for fast moving transportation. In order to provide those high-rates of roaming you would require a large number of APs in close proximity along the track, requiring power + network access at each point. A better approach would be to provide Wi-Fi inside the train and use some type of broadband (WiMax, satellite, etc.) wireless connectivity to connect the users to the public cloud.
