If you want to leave your car behind, or just dont drive, Souillac is well connected by trains (Brive/Toulouse axes) and there are buses to Sarlat at least twice a day (basically to have a connection in between the two train stations which are not connected otherwise) with a single €2 fare!

Souillac SNCF Station information

Time table for the Bus line Souillac/Sarlat