There are numerous routes that have scenic beauty of various kinds. Even a single route, such as one of the transcontinentals--will have many different kinds of scenery, sometimes within just a few miles one another. different kinds of scenery appeal to different kinds of people. Like the USA's motto--out of many comes one. That holds true for scenery. And don't forget cities and other urban areas: they too offer a wide variety. Imagine riding the California Zephyr--Chicago(The Windy City), Denver (mile high--at the foot of the Rockies), Salt Lake(the Mormons, Wasatch Range) , Reno (Biggest Little City in the World), San Francisco (City by the Golden Gate). Also, small town America. There are so many possibilites and things to see.
This is true not just of the California Zephyr, but of the Coast Starlight, Southwest Chief, Sunset, Empire Builder, the Northeast, and many others. Get out and enjoy it all.