Luggage on Renfe

Should be no problem - if you can carry it, you can stow it on board. There will likely be luggage racks at the end of the car to set your suitcase in, and you can proceed to your seat.

Spanish train security isn't as extensive as airport security (you don't have to remove your shoes), but expect your suitcase to be x-rayed before you board - quick and easy.

Should be fine. At the end of most, if not all, train cars is a luggage rack. Smaller bags (airline carry on) can go on a rack above your seat, and small purses/day packs go under your seat. Just like a plane except there is no checked baggage - just load it onto the rack at the back of the car.

As far as security, the high speed renfe trains have you put your bags through a scanner - conveyor belt.

Awesome, thank you very much for clarifying this.

I'd add, only the fast trains x-ray your bags. They typically won't clear you to the platform until about 15-20 minutes before your train, so getting to the station much earlier doesn't help.

For regular trains, just go straight to your platform - like anywhere else in Europe.

As was said, as long as you are capable of getting your bag(s) on and off the train, maybe add "in one trip", it's no problem.

The fast trains have fewer stops but the stops are, at minimum, several minutes each.

Regular trains stop more places. You should be ready to get off when they stop, especially at the small stations where the stops can be very brief.

If you haven't bought your train tickets yet I would suggest you might want to consider selecting seats near either end of the car to be closer to your luggage. There is overhead storage above every seat, but that's more for carry on type bags. Larger suitcase might fit, but they might not. Aside from that, it would be difficult pulling your luggage through the aisles, even the four wheel kind. So, storage at either end of the train car is probably what you will be doing. By the way, a lot of those suitcases look the same, so you might want to distinguish yours in some way. Lastly, I'm not suggesting there is a problem with theft, but in general you might feel more comfortable knowing you are sitted closer to your bags.

Actually good point @travelguymiami... as said, not that there's a problem per se with theft but, me myself being local I wouldn't feel comfortable leaving my luggage too close to an exit nor out of sight. I guess it's a cultural thing, I remember when I lived in London taking the metro from the airport and seeing Londoners leaving their luggage at the entrance of the carriage while sitting a few yards away noting anyone could simply take it and get off without them being able to react on time. That's something I never felt comfortable with.

So Renfe..., when travelling on a commuting train I'd leave the luggage next to my seat, there's no problem there because the carriages are wide enough to move your suitcase as you please -unless of course you're taking it in rush hour!-; and when travelling on a high speed train -where interior resembles an airliner - I'd do as @travelguymiami suggests.

So unfortunately I did already book them. I actually don't remember seeing an option to choose where you sit. Is there any way to change this after booking?

Your seat is already assigned. Your luggage will be fine. Really. Keep your valuables with you.

Not too many people interested in stealing a week's worth of dirty laundry in giant locked suitcase they have to lift off the train.

My only point was - if you are booking tickets on Renfe you can select your seats. If one has an option to select seats where your luggage is in sight which would be towards the front or rear of the train car I think that would be a better choice. This is not meant to imply that there is an issue with luggage (but there are stories on this forum and others describine such incidents). So I think better safe than sorry and the peace of mind knowing you can see your luggage.

Seats face different directions - so even if you are sitting nearer to the end of the car, you might not actually see the luggage racks. Some people will lock their bags to the rack. If you do so, make sure others can use the rack and move your bag around as they can get crowded with lots of bags. Otherwise, just keep an eye on it at the station stops - nothing is going to happen to it while in transit.

Just don't dump your bag in an empty seat. Even when the train is not crowded, it is rude.

Yes, the seats face different directions which are clearly shown on the seat maps on Renfe when you select your seats.
On our recent trip to Spain we took Renfe 6 different times. We clealy knew exactly which seats we were selecting, which row, and which direction. The seat map displays everything, including where the luggage storage is.

As far as I know, you can only see the seat map when you are actually purchasing your tickets. My guess is that if you skip this step, Renfe probably just randomly assigns the seats, but I don't know if you can change them once they are issued. Perhaps someone more familiar with Renfe can elaborate or clarify.

Here is a useful link for all European trains that displays seat maps - incluging for Spain:
http://www.seat61.com/Europe-train-seat-plans.htm#Spain