RENFE phone app

I have just booked a ticket from Madrid to Leon online at home. Renfe will send you a pdf file on your submitted email which will be your ticket. As far as I know you have to print it to present at station. In your case you have to make use of an internet café (Pilgrim's Office at Santiago have one, perhaps experienced members can correct me about this). For more details about buying Renfe Tickets have a good look at this: http://www.tripadvisor.com/Travel-g187514-c80518/Madrid:Spain:Buying.Renfe.Tickets.Online.html. I found it very useful. Take also note about Promo prices. Another thing which you might also consider is taking a flight from Santiago to Madrid. I bought a ticket from Ryanair at €25 including taxes. Hope this helps mate and buen camino amigo.

I know that buying online a train ticket may be convenient (and oddly, quite satisfying), but you can also do it in the Sarriá train station -that is four or five days before Santiago, when you will know your expected arrival date. The Sarria station is 15 minutes from Rua Maior (that is main street, and the Camino).
Also, in Spain, practically any travel agency sells RENFE tickets.
Buen camino!