https://rodalies.gencat.cat/en/linies_estacions_i_trens/index.html?linia=r3

It’s a Rodalies train - you can’t reserve seats and - according to the Reddit page I ended up on when Googling - you just show your pass on board.

If that day you are only doing that Latour to Barcelona trip, you may however want to just pay the relatively cheap fare and preserve one of your pass’s travel days.

RENFE don’t bother loading the “commuter” lines to the timetable databases so these trains do not show up in most planners.

Here is the current timetable https://rodalies.gencat.cat/web/.content/02_Horaris/horaris/R3.pdf

If using a pass you will have to manually enter the journey.

Just as a note, on arrival in Barcelona you will probably encounter access gates. Approach a member of staff and show them your pass, or contact them through the intercom. 

I did this trip on an interrail pass last summer in opposite direction. Using Interrail was no problem at all, even though it indeed does not appear in the app. 

As a note to @Schelte 

You can climb over the gates ;.)

Just back from 8weeks travelling. (global pass)

Our last stay was in Barcelona. We came from La tour de Carol using the R3 (rodalies). Our appartment was close to Barcelona Sagrera - la meridiana, a stop on the R3. The QR code didn’t work (says invalid), no one responds on the button (we pushed it at least 10 times) for help at the gates, so we climbed over (we are with 2 so hadn’t left our bags on 1 side alone) the gates. (so if you really need help you’ve got a serious problem)

Last day in B - we did the same at Sagr-mer to enter, to get of in Barcelona arc de triomphe. To get in there later that day: at that moment there was a Renfe-employee. He first asked us to use the QR code (which obviously didn’t work) and then let us through. 

At the airport there were no gates. 

And add those trips manually to your trip 

Btw: renfe has an app with all rodalies /Rlines schedules in it. This worked quite well (Rodalies de Catalunya)

Well… I’m currently travelling around Barcelona. That Rodalies app kind of works but it doesn't have real time information and doesn't show changes due to engineering works. Not even for works that have been going on since last year. It's more like a different way to view the static timetables...

@rvdborgt - Oh, I just used it for having the data by hand to do the manual entry. But realtime and changes would also be convenient indeed. (had the same experience in Italy - trenitalia app doesn’t show real time or delay, at least not very up-to-date info in the app)

The Trenitalia app did show delays when I last used it but for Italy I mainly use the TrenIt! app, which is faster and easier to navigate.