Why not walk ? Saw many pilgrims walking alongside the beautiful coastline when we were on holiday there in Saint Jean de Luz last month.
It is a distance of about 50 kms from Biarritz to San Sebastian.
Not the latest information but maybe you it gives a clue
24 March 2006, 3:03
It is certainly possible, and should be quite easy. There are, I think, about a half dozen trains a day between the two cities, and the journey time averages around 90 minutes. I did try to find a timetable for you on the SNCF (French railway) site
www.voyages-sncf.com but, as is not uncommon with cross-border trains, the SNCF site was somewhat unhelpful. (Make that totally #^&#*(# useless!)
If you google the Spanish rail site (something like renfe.es I think) you might get more joy from them. If not, the German rail site will almost certainly give you timetables (believe it or not) but you won't be able to book tickets through them.
www.bahn.de
Another option and more recent
1. Re: Biarritz Airport>San Sebastian - June
24 May 2012, 20:11
On the right hand side of the page theres a link titled "Biarritz Airport by Public Transport". I think this should answer your questions.
If your only option is train to Hendaye (Hendaia in Basque) then you can take the Euskotren to San Sebastian. The Euskotren costs 1.70€ and takes a little more than 30 minutes. The Euskotren station is a few feet from the Hendaye SNCF station.