Superliner Is for sure. So far they’ve only been made in brass and Railking. More paint schemes available so it makes things more viable.
If it were me, I would do what was done with the Viewliners, and combine the project with other cars to increase viability. Run the Superliners with El Capitan Hi-Levels. One downside is the Hi-Levels would have to be re-tooled a bit to allow for the modifications made to the prototype to be modeled correctly. That means changing the truck design so it shows the exposed bearing caps, applying appropriate HEP detail, adding marker lights, and changing the diaphragms. That probably wouldn’t be a massive deal though since there’s a base to work from, and not entirely new tooling required.
I think done this way, it could for sure work. Superliner Is have appeared in Phases I-IV and VI, but also some coaches (and possibly coach baggage) have appeared in Surfliner and Amtrak California/CDTX colors. Hi Levels have appeared in Phase I-III, with some coaches making it to Phase IV for Heartland Flyer service. Then of course there’s the Parlour Cars which appeared in Phase III, IV, VI, and now have seen other paint schemes from their new owners (Wisconsin Great Northern for example).
At bare minimum I would order a Parlour Car in Phase VI, three coaches, (one in Phase VI, one in Amtrak CA, one in Surfliner) a Sightseer Lounge in Phase VI, and a coach baggage in Phase VI.