Railways enable people to travel anywhere in Europe.

Personally, if I had only 5-6 days, I would spend it all in Barcelona. You could get away with 3 days if you really paekcd it in, be you would be exhausted. 5-6 days gives you enough time to see the major attraction and take one day trip (Sitges or Costa Brava for beaches, Montserrat for mountain hiking and the monastery, the Dali museum at Figueras, or Girona).It depends on how you like to travel and what you want to see and do. I would say there is somewhat more to do in Barcelona than in Seville (and personally I wouldn't imagine any less than 3 days there). Probably the rail pass isn't worth it since you wouldn't get to too many destinations. Just by direct from the RENFE website.