Considering taking the train from Bordeaux to Sarlat for one night and on to Carcassonne. Any suggestions?
Have you checked the SNCF website to see if, logistically, your plan would work?
Yes. Trains are available.
It's 2-1/2 hours Bordeaux-Sarlat by train and 5-1/2 - 6-1/2 Sarlat-Carcasonne.
Sarlat is charming but a one-night stand there in exchange for 8-9 hours on trains will be disappointing.