I'll take economic umbrage at "don't sit with the cattle". When I traveled from Chicago to SF on the California Zephyr (40-ish hours), a seat was $100 and a sleeper car was over $1,000; for reference, a flight between the two is about $200. Being able to afford such a sleeper car is the sort of luxurious expense that most vacationers will likely prefer to spend on going to a more exotic destination by plane.
(I do recommend the California Zephyr, and second the parent's suggestion of spending the whole of it in the observation car, but skip the part of the route between Chicago and Denver; Denver and onward is where the real action's at.)
If you're buying a sleeper car ticket, you should be comparing it to an international business class flight with a lie-flat seat. Not a domestic flight in economy.
I'd agree only if our benchmark of comparison is "I want to travel while sleeping", but the value of flying is that you get there so fast that you usually don't even need to sleep. The journey from Chicago to SF is eight times longer by train than by plane, and, price aside, the only value proposition is a (very) scenic view; considering price, you can either spend half the price of flying in exchange for two rough nights stretched diagonally across two train seats, or five times the price of flying for two nights of decent sleep.
I did Denver -> SF this past summer and it was absolutely unreal. I was extremely happy I did it, the sights were gorgeous, and it was an awesome time to just sit and think while some of the West's most gorgeous scenery passes by.
(I do recommend the California Zephyr, and second the parent's suggestion of spending the whole of it in the observation car, but skip the part of the route between Chicago and Denver; Denver and onward is where the real action's at.)