Being a railfan in the midwest, before moving to NY want to add a comment about the Western Streamliners. As previously noted most passenger equipment ran mixed under specific agreements. Trying to keep this short, just an overview.
UP Streamliners ran west from Omaha, the Chicago-Omaha portion was handled by CNW until 1955 and then by Milw until Amtrak. Milw changed their passenger paint scheme to UP Yellow and Gray from Orange and Maroon so cars would "coordinate".
The California Zephyr was a joint train run by CBQ, DRGW, WP. It also had a PRR sleeper in the mix. The percentages of cars provided was based on "rail miles/expected passengers".
The CBQ owned trackage to the Twin Cities from there split to NP and GN to the pacific northwest. All of these railroads were part of the "JJ Hill empire". So even prior to BN the trains were mixed consists Chicago to Twin Cities.
CBQ also had their FWD and CS subsidiaries.
The above gives credence to the CBQ Everywhere West Slogan.
ATSF Was no slouch with Western Streamliner service. Actually Initially "fought off" AMTRAK (as did DRGW). However they, along with the fore mentioned railroads ran "through sleepers" from the Eastern Railroads.
Then you have the RI and Katy, also RI and SP that ran mixed through trains.
Also needs to be noted that while SP didn't necessarily run equipment from other roads on the CoastLine they did coordinate schedules and offered "through ticketing" agreements.
Lastly if you really want to run mixed passenger consists just say you are modeling AMTRAK in 1971-73 (anything goes then). 🤣
