Here's a thought: if you have the time, break up the trip by spending a night someplace in the Rockies. The Izaak Walton Inn http://www.izaakwaltoninn.com/ is actually a whistle stop for the Empire Builder. It's a great small hotel in a gorgeous location, right outside Glacier National Park--and September is a beautiful time there.

We took the Empire Builder there and back from Seattle a couple of years ago. Like others have said, it's uncomfortable sleeping in coach, but not awful, and the scenery is worth it. Westbound, you'd arrive around 7:45 pm (after one overnight on the way), so you could get off the train, clean up, get a good night's sleep. The next day you'd have time to do a little exploring (we rented a car when we were there, but there are also bus tours), or go hiking around the Inn, or just sit on the porch and watch trains. That evening, you could get back on the train for the rest of your trip home. That would be very do-able.

It's a little miserable at night, especially if the man sitting next to you smells like canned tuna, but over all, it's not a bad way to spend a few days. I went coach from LA to Boston one summer and enjoyed it for the most part. I spent most of the time in the observation car, which had more room to spread out than my seat. I was traveling alone and so I read a lot and listened to my iPod and watched the terrible movies they showed (The Phantom of the Opera and Along Came Polly and that one where Ray Romano and Gene Hackman try to get elected mayor of a small town in Maine and Gene Hackman is the former President) and looked out the window. The part of the trip between LA and Chicago was the best, because there's so much nice empty land in the west. I enjoyed eating with friendly strangers, though that's not the sort of thing I would normally feel comfortable doing. I was a little achey from not lying down at the end of the ride, and the one night next to the canned tuna man (who boarded in the middle of the night in Ohio or something; you might be safe from that sort of thing east of the Mississippi) was kind of unpleasant. But, really, I would recommend it to everyone.

I have done this, although I broke it up into two segments (spent a night in North Dakota).

It's worth doing at least once. You get to see a lot of the country. There are some surprisingly pretty parts on Montana.

On the other hand, you will not sleep well, so make sure you have a few days to catch up on sleep once you get back.

Not really an answer, but I've often thought it would be nice if they would let you buy part of a sleeper. Getting a sleeper berth is a flat-rate no matter how many people you put in it, so if you can fill all the bunks they're a pretty good deal. I guess those are kind of close quarters to share with strangers (with no bathing facilities), but it's not any worse than a lot of hostel-type set ups.

I would agree breaking up the trip would probably be a good strategy when going coach.