Spain is just something else when it comes to this sort of planning. Away from the main routes everything is classified as cercanias - commuter. It is then compartmentalized into regions and not presented in any consistent or easily searchable way, this all despite one company running most of it.

The first line you are asking about is hidden away under the FEVE heading, FEVE being the name for the narrow gauge routes in parts of Spain. Sadly those lines have a terrible service, just 2 trains each way Oviedo - Santander and 3 Santander - Bilbao. You can get the schedules here: https://www.renfe.com/es/en/suburban/cercanias-feve/timetables

I assume that if the locals use public transport in that area it is going to be bus not train.

From Bilbao to Hendaye in France the services are run by Euskotren, not part of RENFE but run by the Basque country government. These are in comparison well run with decent service on their lines but not valid with Interrail.

https://www.euskotren.eus/en

https://en.wikipedia.org/wiki/Euskotren_Trena

From Hendaye to Biarritz there are regular SNCF services.

It would probably be better just to use local tickets for travel in this area as they are all fairly cheap.

Wow! That’s a pretty comprehensive answer, many thanks Al_G.

It sounds like a bit of a seat-of-the-pants thing around that area. Like you say, if the local trains a quite cheap, then is no big deal.

Thanks again.

R

I’d been looking at a trip in that area myself so just sharing the results of some frustrating searches and interestingly presented Spanish websites.

Have been myself there some yrs ago-the best of the FEVE lines is the inland line (just 1/day!) Bilbao-Leon (ends nearby, last piece with railways bus into town). FEVE is now  part of RENFE and hence passes SHOULD be valid-but a recent report told us he was still charged 50% of fare (with valid pass)-as the machines of the conductors selling the tic have not yet been programmed after 2-3 yrs. and passes are ultimately extremely rare there. yes-BUSes also run-much faster (por autopista) and much more frequent (ev hour or even more)-mostl by ALSA.  (which is Nat. Express-well known to Brits).

In Asturias=the area around Oviedo/Gijon is a more dense local network, interspered with also RENFE cercanias lines.

Note that RENFE Cerc (rodalies in Catalunya) are not spread all over country-rather they are local commuter lines serving the smaller area around all main cities with more frequent service (and NO reserv and all that-usually magn. stripe tickets and gates), they are not at all interconnected to each other.

For railfans a real treat-if you just want to travel quick+easy=its bus.

Note these are NOT real trains, but more trams-with a little more comfy seats and a loo (if open and working) in the car.

FEVE have been accepting (well, sort of) Interrail for 7 years now. Staff training seems to be a serious problem.

Thanks for that, I will definitely put the Leon FEVE line on the list.