While I'm not an expert, I did stumble across this link for 'couchette' in Germany-kinda cool- much more modern
http://homepage.mac.com/wkaemena/FS/DB/LiegewagenCNLDB/
Now I have no idea how much it would cost, but to me, it would a very attractive alternative to the floor!
Couchettes in French or liegewagen in German are basically cars with a compartment somewhat deeper (front to back in the direction of the train) than Superliner bedrooms with 3 bunks stacked against the front facing wall and 3 against the rear. Unless you happen to reserve all 6 spaces, you would be sharing the room with strangers. You get a sheet, blanket, and pillow. The cost had been approximately $25 for quite a while.
I found them very nice, unless you ran into noisy students (although I could sympathize since I would have been extremely excited to be able to travel like that when I was a student). A problem for Amtrak LD distance trains would be the fact that the bunks are fixed; there is no place to sit during the day. Most overnight western European trains depart around 10:00 and arrive around 8:00. So, if I wasn't ready to sleep, I would stand in the corridor with my head not quite out the open window. Quite an experience rocketing along at over 100 mph in a normal, i.e., not high speed train.
Note: Amtrak slumber coaches were something entirely different, just sleepers with smaller compartments. Then there were cars with what I believe were called sections, i.e., upper and lower berths with a curtain separating them from the center aisle. During the day they would become two facing double seats.