What I have been told by several folks in the industry is that no new Superliner could be deployed today due to ADA rules. The current Superliners are grandfathered. I am not exactly sure what needs to change though, but apparently all of the problems disappear if you do not insist on multi-level. Apparently when you have commonly shared facility for the entire train like Diners and Lounges, accessibility to them from all of the train is an issue. MLVs still are OK on Commuter trains since they do not have any such shared facilities. This is my vague understanding from various conversations and may indeed not be entirely correct since I neither have nor claim to have any deep expertise in this area.
Apparently even Siemens had to bend over backwards to specifically change passenger accommodation designs, specially in the area of width of aisles, vestibules and gangways, to meet new ADA requirements. I understand that in some cases they went above and beyond what is minimally required, for example in the area of automatically deployed gap fillers etc.