Are the residents not called "guests" these days?
When I have been on a day train that has stopped at Carstairs I have been surprised by how many people board/alight especially to/from Edinburgh, so I assume it is used as a railhead for the many surrounding villages. Possibly even Lanark.
I would assume the same logic would apply to the sleeper. The only reason I haven't personally is that I tend to end up using the sleeper one way and either day train or fly the other (I live just outside Ayr, an 1 hour drive from Carstairs).
Carstairs is even staffed most of the day.