The DB planner is more up to date than the Interrail website and the rail planner app, although it also relies on what data RENFE send to the European Timetable Centre. You can use it to search for journeys using regional trains only. You then select "only local transport”. That will show regional trains where they exist, although some still require a reservation (the planner will say that), which can be free or cost €4. Do not try to look too far in the future; 2 months in advance is already a lot for Spain. In addition, no suburban trains are in the planner, simply because RENFE don't share their timetables with the rest of the world.

When I was in Spain (a year ago now), I found it very difficult to find any trains which didn’t need reservations except local commuter trains radiating from bigger cities. That was problematic as I could not reserve trains from outside Spain - I tried calling the RENFE international number and they told me to go to a larger station once I got to Spain.

Good tip about using the DB planner or DB Navigator app. I had several RENFE apps installed at the same time, and some of them seemed to show more useful information than others.

The positive was that the person who helped me in San Sebastian station spoke very good English, knew the train system extremely well, and was very helpful. Ditto for someone who helped me at Salamanca station when I’d missed the last train of the day towards Medina del Campo.

Travelling in the South without reservations is almost impossible. It’s just the way it is Spain.

Trains that don't need reservations are very limited:

- All Rodalies trains in Catalonia (including MD, but only Rodalies)

- All Cercanías trains (Commuter trains around cities)

- Feve narrow gauge trains

All the rest do require reservations:

Regional - regional trains, stopping at every stop. These do require a reservation (since Covid, they might have stopped again) but the reservation is free. No seat assignment, so don't ask me why they bother.

MD/Avant - higher level regional trains. Require a €4 reservation including seat assignment (except Rodalies MD, but check just to be sure - particularly if continuing outside Catalonia)

Alvia/Intercity/Euromed/AVE all require reservations, starting at €6.50. Note these can often be made through some European operators too (that I know: DB)

AFAIK there are still regional trains without reservation, e.g. Madrid - Segovia. In any case, RENFE does not indicate mandatory reservations for them.

I was told I needed a reservation for the Zaragoza-Canfranc and Valladolid-Gijón "regional" trains about a year ago. It was free, though, and likely on board staff wouldn't care much (to be fair, they generally are not worth using a travel day for, unless you're taking other trains or have a continuous pass).

Madrid-Segovia might be a special case, as it's in fact an extended Cercanías service. But overall, I'd say it's best to assume they all require reservations and ask at a ticket desk.

RENFE does not specify mandatory reservations for the regional trains on these lines in their timetable data, as they sent it to the European Timetable Centre. But I notice they don't do that anymore either for the afternoon train Badajoz - Puertollano, which used to have mandatory reservations (€4 in this case), so the data they sent may be faulty. (The DB timetable is a good source to see what data RENFE shared with the rest of the world.)

There are a number of offers on rail travel in Spain for 2023. In particular there is “free” travel on Cercanias and MD trains. They work by buying a ticket (Mobile QR code or paper) for 10/20 euros and if used more than 16 times in 3 months you get the payment credited back  (2022 rule 2023 may be a different number and validity period) . 

Currently Renfe is still  only showing the 2022 scheme details, but the Spanish government has annoumced its continuation for the whole of 2023.

The MD scheme is national but the Cercanias ticket is regional, so the Malaga ticket covers only 2 lines but the Barcelona one is extensive. Routes and timetables available here: They cannot be used across regions.

Renfe | The Best Discounts for Train Tickets ▷ Choose yours!

Note the Barcelona (Rodalies de Catalunya) network includes the service to Portbou/Cerberes on the Spanish border.

These are not high speed services and stop frequently and are typical commuter trains - minimum hard (wearing) seats and plenty of standing room for rush hours.

It is difficult to find more info at the moment, but we managed to use the 2022 version on a recent journey to the Costa Del Sol - you save over 3 euros per single journey. We found a lot from the renfe cercanias app.

These tickets are called Abonos Gratis and offer free travel on Cercanias and MD trains. You can also get 50% off a range of MD/AVE trains.

Some info on here on MD/Avant services - you may need to use google translation.

Free frequent flyer pass | Renfe