Train Travel from Barcelona to Sitges

This is the sight you need to book on: https://www.renfe.com/es/en.

For Sitges you can catch the Rodalies line R2S train from either Barcelona-Passeig de Gracia or Barcelona-Sants. You can't book online you just buy a ticket on the day at the station.

Try this Rodalies journey planner for times to Sitges - timetables - but it only shows the next 30 days.

It's both very simple and a little tricky. Like others said, you can leave Barcelona from any of the three stations mentioned. I like Sants, only because I'm familiar with the station. You can buy the ticket on the Rodalies machines. Simply adjust the language to English (or whatever other language you want), then punch in your destination (Sitges). I think you get an option for one way or round trip. I usually do one-way, because I will probably lose the ticket for the way back. The tricky part for me is to find out which Via the train leaves. From what I remember, there are two trains that pass through Sitges - one of them has the destination Vilanova del Getru (I think). I forget the other. So, I always ask someone - "Cual es el tren para Sitges?" They leave really frequently, so I'd advise you to not hurry - just make sure you're on the correct train. They announce the stops. It's about 35-45 minutes and the train station is pretty close to the beaches, restaurants, shops.

Thank you everyone for your responses! You have all been very helpful, and now my travel plan is in place!