Hi Geoff,
Good luck on your trip to Spain. Went all over the country by train (including to and from the uk) last year and had a fantastic time. Renfe takes credit cards, just make sure that you bring your passport with you. I would meet your friend in the main ticket office in the main part of Atocha station, hidden behind the big rainforest thing - there are plenty of seats there, and a member of staff wandering about who speaks english whose job it is to direct lost tourists to the right ticket counter!. Hope this helps.
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El pueblo, unido, jamas seran vencido!