To travel by Interrail from Nice in France to Madrid in Spain you have two main routes. The more important one from Paris and Marseille to Barcelona. The other one from Bordeaux via Hendaye/Irun to Spain.
The high-speed-trains from France to Spain require an extra reservation with Interrail. But you can travel for free using local trains in France and Spain to cross the border. Also the route from Perpignan trough the Pyrenees (Little Yellow Train) to Barcelona is free.
Your travel route could be like that.1 Nice (France) - Madrid (Espagne)
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Trouvez où dormir. Réservation facile et sécurisée.