What I learned, just a little too late, was that all the relevant local/cercanias Rodalies trains use tracks 13 and 14, which share a platform. So, if you are a commuter and have a pass or multiple tickets, you can indeed easily manage a 10 minute transfer from one Rxx train to another Rxx train, because you either get on right where you got off, or you walk ten paces to the other track. There are screens listing the trains right there on the platform.
I learned this because I was standing there watching, after having spent more money (but not a lot more) than I needed on a Euromed that I took for one stop from Tarragona to Sants, then going upstairs to the ticket windows and getting a general ticket for the Rodalies up to Girona.
While I was looking around near the ticket windows, an older man with a cane approached me and asked in a hard-to-follow (for me) Catalan voice if I was going to Girona because he could take me there by car. He had a business card, so he must be legit. I told him in my hard-to-follow (for him) Castellano that I already had a ticket for the R11. He wouldn't take no for an answer, at first, actually grabbing my elbow to usher me towards the street. I found it a little funny, but I probably wouldn't have if he'd been bigger and I'd been younger or a woman. I actually wondered how he knew that I was going to Girona -- did he know me somehow? But then I realized that most any foreign tourist standing near the cercanias ticket windows of Sants station are more likely than not to be going to Girona. If I was more fluent, I would have tried to bargain with him just to find out what an off-the-books ride does cost.
In any case, if I'd gone with him then I would have missed the show on the train, where a family made use of the facing seat rows to put on a full picnic-on-the-fly followed by cards and board games that I'd never seen before. You appreciate the preferente seats that much more when you have the occasional bit of ruido on a local carriage. No one is going to sleep through those ringtones, that's for certain.