Arles to San Sebastian

It is along ride but take the train from Arles to Perpignan.
In Perpignan catch the train to La Tour de Carol.
This yellow train runs this route through the Pyrenees.
But then from La Tour you have to go north to Toulouse to get a train to San Sebastian.

https://about-france.com/tourism/yellow-train-pyrenees.htm#route

The ride on the Yellow Train is very nice; I recommend it if you can put together a routing that isn't too long to fit into your itinerary. This year I traveled from Latour de Carol through Foix, Tarbes and Pau to Bayonne. I did, indeed, end up having to go through Toulouse--and I would have been willing to take buses to avoid the extra train mileage if that made sense, schedule-wise. It did not.

I assume you wouldn't have to go quite all the way to Bayonne before turning south for San Sebastian.

It is a difficult one. I worry that a trip involving the train Jaune would exceed 12 hours, making it impossible in a day.
The fastest route is by train from Arles to Bordeaux, then Bordeaux to Hendaye, then the Euskotren commuter train to S Sebastián. That's already 9 hours.
Going through Spain takes 12 hours and costs much more.

Any chance you can rearrange your route to overnight somewhere? Carcassonne would be interesting, or Toulouse.

Thank you all for the great advice! We’ve decided to fly from Barcelona. If you have any hotel recommendations for San Sebastian let me know.