Have had this discussion before, but . . .
The first thing people blame for a rough ride is the track. But could rolling stock be a factor, and especially difference in truck (bogie) design?
Amfleet has the Pioneer III inside bearing truck. Don Oltmann tells us that the primary suspension on that thing is nothing more than a slab of synthetic rubber between the journal box and the truck frame. The lateral motion is simply the sideways deflection of the secondary suspension springs/air bags between the truck bolster and the car body. The longitudinal motion of the springs is restricted by those tie rods, connecting the car body to the bolster through rubber bushings.
The Pioneer III trucks is not the smoothest I have ridden -- that would have to be the truck on the French Turboliner. But the Pioneer III seems to be a simple design where there is a minimum of rubbing parts to wear, and the only thing to go bad would seem to be rubber bushings by whatever mechanism synthetic rubber dries out and cracks.
The Viewliner sleeper has a different truck, and I have "heard something . . . just, just a little bit!" that the Viewliner is rough riding.
If there is some gulf in the technology of Amtrak and the rest of the developed industrial world, Amtrak seems to prefer the type of truck where the journals slide within truck frame pedestals -- you know, the kind of truck with drop equalizers on the primary suspension and swing hangers for lateral motion. This type of truck seems a lot more complex, with a lot more wear surfaces. But for some reason, Amtrak doesn't like the home-grown Pioneer III, prefering Horizon cars to have pedestal trucks over the Comet cars with Pioneer II trucks (I have been on Comets in Boston commuter service and thought them to ride well). I guess Amtrak doesn't like the European derived truck design on the Superliner I's either, going to pedestal trucks on the Superline II's. The Superliner I's substitute radius links for the pedestal guides, and that type of trucks is an outgrowth of the research on high-speed wheel stability started in Japan and continued in England and other places.
Before someone chimes in that "railroad engineering/mechanical/maintenance people have their reasons and we (me?) are a bunch of armchair railfan engineering people", my poppa worked for GATX as a research engineer back in the day (you know, the people with the tank car fleet), and he explained that the Metroliner had rough riding qualities because the Pioneer III truck was replaced by a pedestal truck at the insistence of the Pennsylvania Railroad for some reason that seemed like "just because" to him. I know us railfans and amateurs can come up with all kinds of ideas, but one shouldn't assume that railroad people aren't falling back on tradition and on what they know and are avoiding improved designs.
So, a passenger in coach can have a pleasant ride whereas someone in a sleeper compartment can be tossed about, and maybe it is the truck and not the track?