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There are several guides on the RENFE website like http://www.seat61.com/Spain-trains.htm#How_to_use_www.renfe.com or the hugely comprehensive
http://www.tripadvisor.co.uk/Travel-g187514-c80518/Madrid:Spain:Buying.Renfe.Tickets.Online.html (see point 22).
You are too far ahead of yourself for a date in March. Simply look to an imminent date on RENFE's site to find what will almost certainly be available, little or nothing will change before June next with the schedules or pricing. And any other website or agency site is reliant on RENFE's input...
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