1. in HIGH season this train often sells out, a few days advance
2. if so, it prob wont sell out at the same for any class, so any pref you have may turn out to be worthless- if thats sold
3. thousands of people have taken this train - if there was nobody, it would not run anymore (that is in fact the case for the erstwhile daytrain - it has ceased)
4. apparently you CAN book online, with patience, via www.renfe.es