yes, trains from Tarragona to Barcelona stop at Sitges. Not sure if you have to change trains at Tarragona from Salou for Sitges, but somebody will confirm or deny Better and faster than the bus Cheap too.
I am 99% sure you have to change at Tarragona but not too much hassle.
Its worth it.... the train journey follows the coast for long stretches. Great scenery and views so get seats on the right hand side of the train going to Sitges.
Also no buffet car on the train so take your own drink and nibbles
the barcelona train stops at stiges but not all journeys you are better advised to take the planna trip that takes in stiges and monserat nless you want to just go direct then you will need to change at castel delfells