I didn't realize the California Zephyr only ran as far as Denver May 1, 1971, and only as far as Salt lake City/Ogden by July 1971.It
Amtrak did not use the California Zephyr name until the train was permanently routed on its current route in 1983. Initially there was daily service Chicago to Denver, and it continued to Oakland three days a week. At first, the train used the former railroad names and train numbers of the Denver Zephyr and City of San Francisco, then it became the San Francisco Zephyr. The tri-weekly D&RGW train from Denver to Salt Lake City was called the Rio Grande Zephyr, and retained the original CZ train numbers.
So there was service all the way at the start, only not on the route that Amtrak had hoped for...