Paris to Nîmes, via Clermont-Ferrand: Le Cevenol.
It seems that SNCF would rather you didn’t find out about this magnificent train line through Les Gorges de l’Allier – as you won’t find it on their website. You have to look on the TER Occitanie website to find the timetable instead.
But it is a real gem of a railway line that is worth seeking out, as the section of the line which passes through les Gorges de l’Allier (following the Allier river as it cuts through this region), 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:
Les Gorges de l’Allier en Train Touristique (Official Website)