An additional point about RENFE/ESpana:
there are REgional trains-just 1/day, still from Barcelona to Madrid and to Valencia-not honestly said the very best or most modern of stock and as there is indeed NO reservations-can get crowded. A very few other branch lines also have unreserved regional trains.
For cercanias/rodalies it is also: most, all major stops, have gates. In the past one could get special tickets for these on showing pass. This is no more, the system has been modernised and one now has to go to counter, show pass with activated travelday!! and they will then remotely open 1 gate for you to pass. On arrival: find an agent with a key for this. In most areas the gates now work with chipcards.
Minor stops may have only limited hours of staff present: in such a case you seem to be out of luck.
The old former FEVE narrow gauge lines along the coast in the north are now also seen as RENFE and should take passes without extra payment. There have been reports that conductors, who sell most of tickets there, look at their machines which have not been updated and tell its 50%-and thus you have to pay half fare. It is unknown to me if InterRail will refund this.
It has also been mentioned by some that to get a reservation at counter, one must show a passday activated for that day. It is not yet known how universal this is.
And the major bottleneck that very often leads to frustration are the 2 daily very long distance AVE hi-speed trains from Barcelona to Andalucia (Granada/Cordoba/Sevilla/Malaga). Often booked out long before. One can do this trip via Madrid and changing there and thus need to make and pay 2 reservations and allow plenty of time to again pass the checks in Madrid/Atocha.
Finally: besides RENFE there are also train systems as Euskotren in Basque country (formerly this was also FEVE)-also goes into FRance (Hendaya). Has quite low normal fares.
In Catalunya there is also FGC which somewhere states it gives discount-but that is also impossible to get. Normal tourists are highly unlikely to use these lines, except perhaps the Ribes de Feria sideline in the mountains.
On Mallorca there is SFM-this has now special higher foreigner fares and you pay simply by bankcard using to pass gates.
Minor notes: Cerbere is NOT in Spain, it is the last stop in FRance going to Spain. Port Bou is then the first stop over the border in Spain. In the past when there was no hi-speed line yet, one always had to go to just over border to change onward, also due to different gauge. The same applies to Hendaye and Irun-except that there are hardly any trains left going over the border there.
TGV run also to places like Narbonne or Perpignan, to shorten the trip in local TER trains.
@Marion
It's a pity that that these information pages are far from complete as to where you can get the reservations, since especially for Spain that's quite limited anyway. E.g. for trains to Spain and within Spain you can also book via DB and SBB. And Eurostar reservations can also be booked via SNCF and NS phone services.
Last but not least: pass reservations to Spain are available in electronic format. As we know, SNCF sell them this way. When are you planning to change this reservation to electronic format? That would really help a lot of people.
