Paris to Nîmes, via Clermont-Ferrand: Le Cevenol.
SNCF would rather you didn’t find this train. You have to look on the TER Occitanie website to find it. But it is a real gem that is worth seeking out.
The section of the line which passes through les Gorges de l’Allier is one of the most scenic rail journeys in Europe.
There is also a special tourist train which operates between Langogne and Langeac.
Recent articles covering this route:
Current. Les Gorges de l’Allier en Train Touristique (Official Website)