This is FAQ nr 12345 - or so. And also one of the most wanted (and hence asked) rides and also THE major bottleneck, for being told ´completo´. The info on howtodo is -of course- readily in the general info, which will tell you that, oh dear, fear of big fears'in ESpana where RENFE runs the trains in old-style state-official manners, you have to do that in person by talking to another real person working for RENFE, sitting behind a counter. There are a few sideways, but these also take effort. They also will not even think about it if its too early (normally about 2 month advance).

ALT overview of seewulf on howtodo RS by yourself-cheaper too-via railways-will give same info. Dear anna fra sverige still on trip or just returned back home with dotter, greets all new persons here with the link to that.

ALT2. ON seat61com - Espana- you can find a fone nr in UK for those who think this serious matter cannot be left as such-for a guy who seems able to arrange it-for a considerable extra fee, reduced for passholders (but still more as the RES as such cost in ES).

ALT3: use the hourly AVE first to MAdrid, then the other line to SVL-much more chance, longer ride, double cost. Take an hr extra to admire that Atocha estacion de trenes-well worth it-then after instruct BR ( or Virgin or whatever it is who does it this week there) on how to beautify the stations!

Here are the known ways to book reservations for domestic trains in Spain: