As dlagrua wrote above, a roomette that you are interested in may be only partially available between your boarding/deboarding stations.
For example, you may want to travel in a roomette Chicago to Seattle. But one has already booked Chicago to Minneapolis, effectively 'blocking' a Chicago Seattle trip for that particular room.
It is not out of the question to book a room at has 1 or 2 'shorts' where you occupy a different roomette for part of the journey and switch along the way. An alert SCA would simply have you stay put and put the new passenger in your 'move to' room if OK.
About 10 years ago, to ensure I had a bed LAX-CHI that I had to rebook after a last minute cancelation in the prior leg of my trip, I ended up in 3 different rooms for my Los Angeles to Chicago trip aboard #422, the thru Sunset/Texas Eagle sleeper.
I ultimately booked LAX->ELP, ELP-> DAL, then DAL->CHI, each in a room. ELP‐>DAL was the family bedroom.
I had a bed all 3 nights. That's all I wanted. I think it was $6-700 more than I had booked originally. Any port in a storm, and all that.
