Barcelona and Madrid are both great cities. I feel very lucky to have spent a lot of time in Spain. Some other cities to visit: Salamanca, near Portugual, has beautiful architecture. The Basque country, too. San Sebastian is possibly the most beautiful city in the region, but Bilbao is worth visiting for the Guggenheim museum and old town, and the little seaside village of Hondarabbia. Outside Madrid are towns like Segovia, Toledo, and Avila, Toledo is probably the most famous. And Andalucia in the south, Grenada for the Alhambra, and Cordoba is very charming. Seville is very touristic but the old quarter is fantastic and you have to visit the bull ring for a tour! As for taking the train, I found buses to actually be better. They are quite cheap and reliable