And for clarity, it's the red/white "Cercanías" branded services that are RENFE operated. They are just about the only trains in Spain you don't need a reservation for. Lines C1 and C10 run between the city centre and airport, but they only serve T4. Similarly to Heathrow, T4 is quite some distance from the other terminals so you need to take a shuttle bus to transfer.FIP coupons are valid on RENFE services but NOT on the Metro.
You'll probably need to get someone to open the barriers though.
There are also reservation-free regional trains here and there, and in Catalonia, MD trains don't need a reservation either. But in some regions, there are only trains with mandatory reservations indeed.And for clarity, it's the red/white "Cercanías" branded services that are RENFE operated. They are just about the only trains in Spain you don't need a reservation for.