The California Zephyr is a long-distance passenger train operated by Amtrak between Chicago and the San Francisco Bay Area (at Emeryville), via Omaha, Denver, Salt Lake City, and Reno. At 2,438 miles (3,924 km), it is Amtrak's longest daily route, and second-longest overall after the Texas Eagle's triweekly continuation from San Antonio to Los Angeles, with travel time between the termini taking approximately 521⁄2 hours. Amtrak claims the route as one of its most scenic, with views of the upper Colorado River valley in the Rocky Mountains, and the Sierra Nevada. The modern train is the second iteration of a train named California Zephyr; the original train was privately operated and ran on a different route through Nevada and California.

During fiscal year 2023, the California Zephyr carried 328,458 passengers, an increase of 13.1% over FY2022, but down from its pre-COVID-19 pandemic ridership of 410,844 in FY2019. The train had a total revenue of $51,950,998 in FY2016, the last year that route-specific revenue data was given.

Prior to the 1971 creation of Amtrak, three competing trains ran between Chicago and the East Bay, with bus connections to San Francisco:

Railpax (renamed Amtrak in late April 1971) originally intended to revive the California Zephyr as part of its original route network, using the Burlington Northern (ex-CB&Q) east of Denver, the DRG&W between Denver and Ogden, Utah, and the WP west of Ogden. The California Zephyr route would serve more populated areas (including Denver and Salt Lake City) than the Overland Route, would run through rural communities that lacked good highway access, and could attract passengers to its scenic routes.

However, since the WP had shed the last of its money-losing passenger service by terminating the California Zephyr, it was not eligible to participate in Amtrak's formation. On April 12, 1971, the WP refused to cooperate with Railpax, and the SP route between Ogden and Oakland was chosen instead.

On April 26, the D&RGW elected not to join Amtrak. The contract specified that Amtrak could later increase service, and D&RGW feared that would crowd its single-track mainline that competed with the UP's double-track route. The D&RGW chose to operate the Denver–Ogden Rio Grande Zephyr. Amtrak scrambled to piece together a Denver–Cheyenne–Ogden routing on the UP.

Between the spring of 1971 and the summer of 1972, passengers traveling between Chicago and Oakland would have to travel on two different trains: the Denver Zephyr, which operated daily between Chicago and Denver, and the City of San Francisco, which operated three times a week, between Denver and the San Francisco Bay Area. Eventually, however, after several false starts, Amtrak consolidated the two trains into one, dubbed the San Francisco Zephyr, homage to both the California Zephyr and the San Francisco Chief, between Chicago and Oakland. The Rio Grande continued to operate the Rio Grande Zephyr between Denver and Ogden. The San Francisco Zephyr and the Rio Grande Zephyr were scheduled as to facilitate a connection between them in Ogden, but not Denver.

In July 1980, the San Francisco Zephyr was outfitted with new bi-level Superliner I passenger cars built by Pullman-Standard – one of the last Amtrak's western long-distance trains to receive them – (already running with such cars were the Southwest Limited, Desert Wind and Empire Builder), and began exchanging through cars at Ogden with the Seattle–Chicago Pioneer and the Los Angeles–Chicago Desert Wind. Between Ogden and Chicago, the Zephyr, Desert Wind, and Pioneer operated as a combined train.