To put it bluntly, Amtrak's current on-train WiFi sucks; even if you don't mind sharing 10Mbps with hundreds of passengers, there are significant gaps in coverage. Relief may be on the horizon, though. The mass transit company is soliciting bids for a trackside WiFi project that would deliver at least 25Mbps across whole routes. The improved access would both keep you connected for more of your trip and let Amtrak lift some of its restrictions on streaming media and other bandwidth-heavy tasks. If all goes according to plan, you might not have problems watching an online movie (or at least, listening to online radio) during a lengthy journey.
Just be prepared to wait for this much-needed capacity boost. Whoever wins the bid still has to construct the near-track WiFi, and it won't amount to more than a "proof-of-concept" that runs between Boston and Washington, DC. The faster speeds will only expand to the rest of Amtrak's Northeast Corridor region if the trial run shows that it's feasible. Until that happens, you'll likely be stuck using slow data on high-speed rail.
[Image credit: Cliff, Flickr]