Small knife on trains

I don't believe it would be a problem. For the faster trains they do have security points and bags are run through an xray machine, but I did not see anything listing prohibited items like you have on a plane. Given the commuting and basic transport nature of the trains, people are hauling all kinds of things with them.

I had a knife (small folding pocket type knife) in mine, no one batted an eye, or maybe they just never saw them, or even cared.

I always buy a small knife at a market for cheese fruit salami etc. Never pay more than €2 never been confiscated on a train but I can afford a replacement.

Oh my goodness, we just had a problem with this! We purchased two souvenir pocket knives with handles made from a bullhorn in Ronda. Probably 3-4" blade. Took a bus to Malaga (no luggage scanning). When we were boarding the train in Malaga to head to Granada our bag was stopped and we were told we the knives weren't allowed. They let us through this once they said.
But bags were screened on another train trip from Granada to Madrid and no flag. (we were on pins and needles!)
So maybe it depends on the person screening? I'd bring something inexpensive and take the chance I guess. Good luck!

If it's in your pocket, it won't be screened. Only the bags go through screening. You may be asked to remove coats/jackets for them to go through screening.

If it's in your pocket, it won't be screened. Only the bags go through screening

A chance this might cause more of a problem, a number of the checkpoints also use the handheld wands to check pockets. Had to take out my phone a couple times to show.

Security for the faster trains is spotty, in three weeks we experienced wands and x-ray, x-ray only, and at some small stations, no security. It is far from what you would expect at an airport, I do not recall ever seeing anyone stopped or questioned, or bags searched. Technically, if you read the fine print, they do prohibit "sharp objects" and "edged weapons" which "sharp objects" is pretty wide open (a pen or pencil?), but I suppose that allows personnel latitude to make a decision, much like the limits on number and size of bags allowed.

Like I said, we had a small folding knife, plus some plastic silverware, I suppose other items (a corkscrew for one) that would qualify, and never a second look. I suppose if you are hauling large kitchen knife, or a hunting knife...that might be a problem.

Bottom line, go ahead, pack it while there, don't get anything you can't afford to lose, and enjoy.

Which train? everybody else is assuming you're referring to a high-speed train -where indeed you and your baggage will be screened- but not with the other trains, there are no security checks in the rest of the trains.

Having said that, legally speaking, one cannot carry any knife over 11cm (4.3 inches) in public.
Source: https://www.guardiacivil.es/es/servicios/armasyexplosivo/controldearmas/clasifica_armas/armas_prohibidas/index.html#:~:text=Navajas%20cuya%20hoja%20exceda%20de,del%20mango%20hasta%20el%20extremo.

Still, displaying and using in public a "small knife for cutting your chorizo" shouldn't be a problem.

We will be taking a train from Logrono to Barcelona, Barcelona to Valencia and then from Valencia to Sevilla. I will purchase an inexpensive knife and take my chances! Thanks for all the feedback.