Estartit has brilliant sand. Best I have even come across for sand castles and sculptures.

I'm not the biggest sandy beach fan, but even i ventured onto the Estartit beach this year, and even swam in the sea (first time in the 10 or more time i have been there!! )

Great beach for kids and it slopes very gently out ... i could practically walk halfway out to the bhoy marking the end of the swimming area

fiona,
coming from barcelona by train the place you are thinking off must be one of the smaller towns south of calella.
i seem to remember being in sant pol de mer and canet de mer and the beaches were better than those further north.
i think sant pol was the place that had a great seafood restaurant right at the beach.it is on the train route.
i never got off the train as far south as mataro so don't know what that beach was like.
hope this helps.