I believe that the name Zephyr was applied to any of several passenger trains run by the Burlington Route and your cars should look good with a Burlington engine. The California Zephyr was a specific train that started in Chicago on the Burlington Route and switched to Denver and Rio Grande Western and then switched again in Salt Lake City to the Western Pacific and then traveled through the picturesque Feather River Valley in the Sierra Nevada mountains and on to Oakland. So that gives you three different liveries to choose from. The California Zephyr was operated over twenty years between 1949 and 1970. Much later on Amtrak brought the name back, but your cars probably don't say Amtrak on them.
I looked into it and found that in 1997 MTH did make some O27 California Zephyr cars that have numbers on the sides without any names. They are MTH Railking California Zephyr Passenger Cars 30-6050, 30-6051, 30-6052, 30-6053, and a 2 car set in 1998 30-6054.
