When are you travelling?

You picked the two most Interrail unfriendly countries out though, with high reservation fees and limited flexibility for the long distance high speed services.

In France you can avoid those still, by using the regional trains or TERs, but you need much more time. But you’ll see more of the country (about 10 or 20 EUR per seat for a domestic TGV). International TGVs or Thalys are pricier, but they can be avoided. Just ask here if interested.

In Spain you have to reserve practically everything, except suburban trains (reservation cost varies from 4 to 10 EUR per seat in second class).

You can’t book to far in the future in France and Spain.

For France: see this page: https://www.sncf-connect.com/aide/l-ouverture-des-ventes, now until september ‘3

In Spain: impossible to book online. Book at the ticket office in Spain, or a ticket office in Germany. About 2 months before departure bookable I think.

Please read this topic attentively, as it contains a useful guide to book seat reservations, via all other ways than the Interrail website:

Make sure to compare ordinary advance tickets together with the total cost of a interrail pass day + reservation cost per train.

I'd add that booking seats a few days in advance or as you enter Spain won't be a problem. No need to buy weeks in advance, except for the (expensive) Thalys and maybe the TGV down south.

Btw if you'd like to save a bit of money, take IC trains to Lille and then a direct TGV to Bordeaux

Thank you - this is all very helpful - so far I’ve looked at Trainline and Omio for train times and connections - although I still have a lot of searching  and comparing journeys to do I am confident i will make it to Dax. Have to be there on 7th or 8th August.