Madrid to Sitges

Take a high speed train from Madrid to Barcelona and take a suburban train to Sitges. There are three operators on that route. Iryo and Renfe both offer tickets that include free transport on the suburban networks at both ends. So that means using the Cercanias in Madrid to get to the main station, and then the Rodalies in Barcelona to get to Sitges is included. No need for an extra ticket. The R2 to Sitges runs 4 times per hour, so you just hop on the next direction Sant Vincenc de Calders after you arrive in Barcelona...

Book either on www.renfe.es on www.iryo.es. Or use trainline, where you can easily compare the prices of the different operators.

There is a third operator on the route, Ouigo, but they do not include free travel on suburban trains in their product, so you would then need to buy an additional ticket to Sitges when you arrive in Barcelona.

Thank you so much for your comprehensive reply! Looks easier than I had thought. I plan on having everything booked by the new year. Thanks again.

Unless it has changed, the "included" free transport mentioned above is ONLY within Area 1, which does not include Sitges, which is in Area 3. Thus, adding to @WengenK response: once you arrive at Sants Estació -the main hub in Barcelona-, regardless of which fast speed train you took, you'll need to go one level up to the main hall and purchase a ticket to Sitges (it's under 5€ p.p.), then descend again to the appropriate platform and take the commuting train to Sitges. There are several per hour.