I gather that it's only available on the Acela at present??? I'd love to have it on the AT, but that's probably way too much wishful thinking, and too difficult logistics wise, along the CSX route.
WiFi is available on the following trains
listed on the website. Well, its available until several hundred other people try to use it at the same time and when the nearest cell phone towers can handle the data load from a passing train.
In the past several years, Amtrak has added WiFi to the Acela, the Amfleet I coach and café cars, the Surfliners in CA, the Talgos on the Cascades corridor. WiFi equipment has not yet been added to the Amfleet II, Superliner, and Horizon cars. There was a item in an Amtrak report about adding WiFi to the AutoTrain lounge cars, but that apparently has not happened yet.