Some, not me, might suggest it’s a track maintenance issue. Lots of bends, slopes, trees, rocks and other stuff like that. Some, definitely not me, might suggest it’s the wear and tear caused by hordes of Pilgrims with their overloaded backpacks and overwhelming expectations.
I think it’s subtle combination of both. It was a rural backwater line getting kids to school and granny to market until some irriot designated St Jean as the starting point of the road to Nirvana. So now there’s enough passenger demand that SNCF are obliged to keep the line open and nowhere near enough to pay for the investment required. They are keeping the local bus drivers in steady employment though