My daughter is traveling from Chicago to Flagstaff later this month. She just talked to Amtrak yesterday. Your information is incorrect. There are no vaccination or testing requirements for passengers on this route. Passengers are required to wear masks.
She returns on Jan 4! :(
I got it from their own website a couple of months ago. Now it may have changed but it clearly stated that vaccine cards (or whatever they’re called) would need to be presented and that masks are mandatory.