Even though you may be traveling with a Eurail Pass, don’t forget that you have to pay for your reservation before any major train travel. The price varies as I found out the other day when I booked my reso for Granada to Madrid, it was ten euros. The other resos I’ve booked so far have been 4 euros each. When you make these, you get an assigned car and seat, which will be different on your ticket in each country, but for Spain it is Coche (car) and Plaza (seat) or you may see Asiento too. Sometimes the number will be followed by a “V” for ventana (window), which is what I prefer, but I have forgotten to specifically request it in the past and have just got lucky. Usually though, if the train is not full, it is okay to switch seats in the same car, I’ve seen folks do this before.