I think every Amtrak service west of the Mississippi uses Superliners except the Cascades, River Runner, and San Joaquins (Amtrak California has some Superliners in captive service along side the California cars). I also wouldn't be surprised if, besides Chicago, a lot of Amtrak's daily maintenance facilities out west don't have room for taller equipment.
WASHINGTON – Amtrak is taking a major step towards improving overnight, cross-country train travel by issuing a formal Request for Proposals (RFP) to railcar manufacturers to begin the replacement of Amtrak’s current Long Distance fleet.
https://media.amtrak.com/2023/12/amtrak ... n-service/