Hello, I've done one night in coach a number of times, so here's some tips:
- If the Amtrak website shows your train as 80% full, all that means is that for at least one station-to-station segment during the train's entire run, occupancy will be in the range of 80-90%. That doesn't mean the train won't be 80-90% full for numerous station-to-station segments, but it also doesn't mean it will. If you're taking the Southwest Chief from LA to Chicago, generally there is heavier ridership from LA to Albuquerque and Kansas City to Chicago, with a dip in ridership from Albuquerque to Kansas City. Of course this is dependent on particular trains, time of year, etc.
I have ridden a "100% full" Lake Shore Limited train from New York City to Chicago (end to end) that was probably 50% full by the time we reached Chicago.
- If you have a row of seats to yourself, it would theoretically be possible to sleep on the floor between rows of seats. However, you would need to be smaller in stature. Your car attendants may or may not allow it; to be honest I've never seen anyone try it before. I can say, though, that the attendants will not allow you to sleep on the floor in the aisle or in the handicap-accessible area.
- Bring some towels or put your packed clothes in some plastic bags or garbage bags to act as lumbar support in your seat. I've been doing back PT for a couple years and this generally works for me for one overnight.
- If your train has an observation car (this isn't likely given Amtrak's equipment constraints), some conductors allow people to stay overnight in the observation car; some do not. It's hit and miss. However, I doubt they would allow you to occupy the floor.