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 511⁄2 hours.

[3] 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.

[7] Prior to the 1971 creation of Amtrak, three competing trains ran between Chicago and the East Bay, with bus connections to San Francisco:[8]: 136 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.

[8]: 136 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.

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.

Amtrak re-routed the San Francisco Zephyr over the D&RGW's Moffat Subdivision between Denver and Salt Lake City, its original preference from 1971.

However, this required a complicated reverse move along street running tracks to reach the wye at West Oakland.

[13][14] Frequency was reduced to tri-weekly in October 2020,[15] but was restored to daily service on May 24, 2021, after additional Amtrak funding was included in the American Rescue Plan Act of 2021.

Upon leaving Chicago Union Station, the train travels along the Metra BNSF Railway Line, with an intermediate stop in Naperville, Illinois.

From Omaha, the train travels overnight through southern Nebraska and northeastern Colorado, making a morning arrival in Denver.

Leaving the Moffat Tunnel, the tracks then follow the Colorado River from Winter Park Resort to Ruby Canyon, west of Grand Junction, which is also where the train enters Utah.

The line empties out into the California Central Valley, and then runs along the San Pablo Bay, with stops in Sacramento and Davis.