Wouldn't normal cell service suffice as well? Unlike in an RV, you'd presumably never be too far from civilization.

I haven't taken this route specifically, but a lot of Amtrak routes don't follow roads and don't have great cell service.

Almost all long distance Amtrak routes deviate quite far from civilization. Once you’re out of the cities and away from a highway you get cell pockets.

If you can afford a private rail car I'm sure you can also afford a peplink router with dual wan interface, and one of the two way going to starlink, the other to a cradlepoint LTE modem and antenna system for T-Mobile or Verizon.