I took the eastbound SW Chief last summer, and was "up and about" during the time that the train was running across a particularly nasty stretch of tracks at what seemed to be a higher than normal rate of speed. Not being a "railfan" I have no idea of what track we were on, or what speed we were going - I can just tell you it was breakfast time the morning after the day we left LA - we were on-time, and out in the desert somewhere.
Many of the elderly passengers and people with kids (who seem to be Amtrak's main market segment on the western LD trains) were having a rough go of it, particularly at the point where the cars joined together. I wasn't particuarly bothered by it, but the elderly lady across the hall from me was very scared about a fall (although I have to say she was a real trooper - we were in the sleeper that gets put behind the coaches, and she made her way to every meal through all the coaches - despite the attendant offering to bring her her meals)
I don't know if it was the 90mph section, and if slowing it down will help smooth things out, or if the Superliners need better shock absorbers or something else, but I really do think that it was a bit much for a lot of the passengers.