Biarritz - Hendaye : regular TER service.

Hendaye - San Sebastian : Euskotren every 30 min. Interrail/Eurail not valid -> tickets 4€ (buy at the station)

Walk across San Sebastian, make a break too :)

San Sebastian - Madrid-Chamartin : a few trains

Thanks!! Will i need to make seat reservations from san sebastian to madrid?

Yes, you will. By the looks of it it’s a Media Destancia train (San Sebastian/Donostia to Madrid). Almost all trains in Spain need reservation, save for Cercanias and other local trains. You need to get them at the station, only AVE (Spanish High Speed train) reservation can be done online alas.

The 15:17 ARC is available online, at least last time I checked. By all means get a reservation in advance if you can.