Hi, almost all trains in Spain have compulsory reservations. 

I would not suggest to travel to spain. 

I would also suggest to look into this question about the same topic:

I think you’re going to accept that you’ll have to pay the supplements for this itinerary. Even if there are busses you’ll have to pay and this will be more than the train supplement! 

There are some reservation-free trains on part of that route, e.g. on Valencia - Barcelona, but not everywhere I'm afraid. 

And in such cases there will be only 1 tren/REG as RES-free-mostly an old type of suburban set with just hard plastic seats where as the other trenes run more often and are more comfy. But that is only the very last of all sectors. YOu did not very clearly state for when, but if this is OUT of main season, do not fear too much-for the NON-AVE=superfast trenes the supplt is 4 or 6€+ a time consuming visit to a counter-so try to combine as much as possible in 1 go-and hope+pray for someone who knows how to pull the ropes for it.

BTW-IF you can commit to fixed bookings long before, just check what normal advance tix would cost-it may not differ very much with a pass with days to use.

For IN ESpana-which has a very wide and far more outreaching coach(BUS)Netowkr as even the UK: FLIX is just a minor one there and mostly aims at bringing people in+out of country. There is not 1 general national company, though many are by Avanza or ALSA, which is owned by the GB Nat. Express! Fares are NOT overly cheap as govmt regulated. There are usually ample discounts for seniors (tercera edad).