Yes, I'm guessing there are others here who might have done it with a bit more advance notice. Prices seem reasonable in that they appear to include all that you need (starting at $115.) The higher price points must get you into a nice room in one of the vintage sleepers.
It's a great ride by the way, esp. in the daylight hours. It's partially duplicating the daylight run of the old Desert Wind, except you only get to go to Kelso rather than Las Vegas and on up the UP to SLC. I have some very good memories of Amtrak's old #36/35...Afton Canyon is quite spectacular; accessible only 3 ways-- rail, true 4 wheel drive vehicles/motorcycles or with boot leather. There are some old talc mines still visible in there, high up on a couple of the ridges. The UP conductors used to point out those mines to the Amtrak passengers.