he original California Zephyr was a deluxe passenger train that ran between Chicago, Illinois, and San Francisco, California, via Omaha, Denver, Salt Lake City, and Sacramento. It was jointly operated by three regional carriers: the Chicago, Burlington & Quincy (CB&Q), the Denver & Rio Grande Western (D&RGW) and the Western Pacific (WP) railroads. Each carrier contributed to a shared pool of stainless steel Budd passenger equipment that made each entire trip, while each used their own colorful diesel consists for their respective portions of the route.

The California Zephyr was conceived after World War II to compete with deluxe transcontinental trains of competitors Santa Fe and Union Pacific. The CZ debuted as "the most talked about train in America" on March 19, 1949, with the first departure the following day. Because the Rockies and the Sierra Nevadas adversely affected its schedule, the three railroads knew they could not compete with parallel trains based on time alone; as such, the routing was billed more as a scenic land cruise and was scheduled in such a way as to pass through its most spectacular scenery on its route in the daylight. This approach proved profitable for much of its 20+ year operation, but like all passenger service it could not compete in the West automobile travel and airline schedules.

The original transcontinental California Zephyr ceased operation in 1971 with the advent of the national Amtrak network, although the D&RGW continued to operate its own passenger service — dubbed the Rio Grande Zephyr — between Denver and Salt Lake City until 1983, using some of the original equipment. Amtrak had intended to include the original route in its debut offerings, but the Rio Grande's choice to hold out forced the new carrier to route its Chicago to San Francisco trains around Colorado via the Union Pacific — eventually titled the San Francisco Zephyr.

In 1983, the Rio Grande gave up on its regional passenger service, and a second iteration of the California Zephyr was created by Amtrak to operate over most of the original route. Today's CZ uses the original routing between Chicago and Salt Lake City and on to Winnemucca, Nevada. From there west, the current CZ follows the Union Pacific's Overland Route through Reno and across the Sierra Nevada Mountains. The original CZ followed the Western Pacific's scenic Feather River Route via Gerlach, Nevada, and Portola, California. Today's trains, numbered Amtrak 5 and 6, make use of the carrier's equipment consists typical of the western routes based in Chicago: a pair or trio of locomotives leads a baggage car, a crew dormitory, two or more sleepers, a dining car, a Sightseer lounge, and 2 or 3 coaches. All equipment but the baggage car are in the double-level Superliner series.