If you are seriously interested in travelling in a sleeper for the comfort and bed, look into getting ahead of time. Auto Train is very popular and frequently sells out. Nationwide, sleeper occupany is up 30% over last year, so less rooms are available on a last-minute basis.

For Auto Train specifically, I believe any last minute upgrades are processed at the station ticket counter. I could be wrong, but I don't think Conductors on Auto Train have any ticketing duties. All they are there for is the operation of the train. Both times I travelled, my ticket was collected upon check-in at the ticket counter and issued a boarding pass to show the sleeper attendant.

I think you will really enjoy Auto Train, especially if you have a room. The service is the best run of any Amtrak train. The station in Lorton is new and very nice. Onboard, the equipment, crews, and food are all excellent. Let us know how the trip goes!