It is technically possible to arrange some sort of shared room when on board. Getting a conductor to actually do it is a different story. What you would need to do is find your companion, and have them book an “Open Sleeper” ticket; it shouldn’t have any additional costs with it. It
This is a good question that should be asked to an Amtrak representative.
Technically it would not occur additional fees, since the room is already paid in full.
According to stories I’ve heard, passengers have the option to upgrade en-route by asking the conductor; the room fare is then calculated according to the remaining distance at the current train position or passed stops.
Credit:stackoverflow.com‘