“Northern Spain” is rather a big region.

If you’re talking about Euskotren, largest train network in the Basque country, between Hendaye-San Sebastian - Bilbao, just get normal tickets. They are very cheap, only a few Euros, and won’t even come close to the cost of a pass day.

Euskotren trains are frequent, but not super comfortable. See it more as suburban or metro rail. For longer distances between Bilbao and San Sebastion, I’lld recommend taking a bus.

Indeed, Euskotren is not part of Interrail/Eurail. Their trains are the fastest connection in the Irun-San Sebastian-Bilbao region. Tickets are at a fixed price and usually not too expensive, so wouldn’t justify using a travel day of your Eurail pass.

Renfe (which is included) runs very few trains in this region with not so great connections. On the other hand if by Nothern Spain you mean more the northwest (Santiago, A Coruna, Leon etc), Interrail is valid on those routes.

I’d advise you to check train times beforehand and bear in mind that pretty much all trains in Spain require an additional seat reservation (5-15€) for Interrail users that can be hard to come by. You’ll have to do the maths.

A good guide to making reservations: https://www.seat61.com/interrail-and-eurail-reservations.htm#Spain

There’s also the Renfe pass which works a little bit like Interrail except reservations are included. Like Interrail it only includes Renfe trains. More info on that: https://www.seat61.com/train-travel-in-spain.htm#Railpasses_for_Spain

But most of the time, for Spain I find it’s easiest to buy individual tickets.

Thank you so much! This information is very helpful.