The California Zephyr is a long distance Amtrak route that goes from Chicago to Emeryville, California. Along the way, it passes through Iowa, Nebraska, Colorado, Utah, Nevada before reaching California at Donner Pass.
As the route and schedule currently stand, The California Zephyr is the best scenic route in the Amtrak line. Most of this is based on the six to eight hours between when it leaves Denver and reaches Grand Junction, Colorado. The train is scheduled so that it goes through the most scenic part of Colorado during daylight. This includes the spectacular Glenwood Canyon. Later on in its route, it also crosses the Sierra Nevada into California, which is also great scenery. (These times are for going from east to west: the west to east travel is not quite as well scheduled, but still affords the traveler many day time views of the most scenic parts).
As a mode of practical transportation, the California Zephyr presents some problems. As with many Amtrak long distance routes across the West, it faces the inherent problem that the area is sparsely populated. The four major population centers it connects, Chicago, Denver, Salt Lake City and the San Francisco Bay area, are separated by somewhere between 600 and 1000 miles each. And many of the areas it goes across are very sparsely populated with almost no local traffic: Northern Nevada being a prime example. Outside of touring, the 48 hour long journey from Chicago to San Francisco is not a practical decision for many travelers.