It works in Spain, although it's best done at least one day in advance, since trains can get full. In Madrid it can also be very busy, although it's not yet tourist season.

If you're still in Germany: Deutsche Bahn can also sell these reservations either by phone (030 5512, collect at DB ticket machine) or at the ticket office. You do need to have the train number ready; you can look it up via the RENFE website or www.raileurope.com.

Also take into account that the rail planner app is very incomplete for Spain, since Spanish railways don't share their complete timetable with the rest of the world. Check the RENFE or Rail Europe website for Spanish timetables.

You can check-in espanol (even though it says in /en/ but it will turn to /es/) on renfe.com, viajar, set to solo ida=one way, then choose from/to and date. It will show all trains on that day and do not be surprised if even 3/4 days ahead some are listed as ´completo´. Trains that you cannot reserve (not on this line) will have a note telling that you cannot book online and have to buy at estacion, these tix are often with magn. stripes or not cards. The Easter holidays start then for many and thats a big travelday.

Perhaps you could check-as this line has just been extended as AVE a few month ago-if there are still some of the 15€ special tickets, then this is cheaper as passday+REServ.

Currently in Espana -no pass though, have another tarjeta, and the experience with RENFE is dreadful till now.

Dreadful in which respect? (Just curious)