Go to THIS site: seat61.com has all the info+booking details for normal tix.

You CAN use-of course, no need to even ask, use also a pass, but for the most direct routing there will be hefty supplms to pay and also very hard to get REServ for final leg INside ESpana. Often noted here as most difficult country to use a pass in. You can save on these supplmts by using local trains=takes longer, more changes, frightening for some unused to it. Hence more advisable if you just not want the fastest direct route but also take chance to visit more interesting places along he way.

If you only will be making the trip to Spain and back it might be worth looking at buying normal tickets instead of an Interrail pass.

Thanks for the replies.

Sorry, I’m completely new to this and don’t know much about Interrail. Is it not valid for long-distance TGV trains?

It is valid but you will have to pay a supplement/reservation for it (domestic between 10-20 euros, for international TGV it may vary).

Thanks.