Possibly not, simply because the Office workers won't be going into Barcelona to work, and you will be going the opposite way to everybody else, who mainly will be coming into Salou from Barcelona and other towns on the way...but I can't say this as a certainty.
Edit...I might be too late with my reply :( If you did go and it wasn't too busy, can you let us know for future reference please? Thanks!
Edit again! are you going tomorrow?

Hello,

Sorry just looked back on here and noticed your reply. I did travel to Barcelona on the Saturday and the train was a lot quieter than anytime I gave caught it midweek. In fact it's the first time I gave got on at Portaventura station and been able to get a seat all the way to Barcelona. So yes I would definitely go on a Saturday again.

Thanks for taking the time to let us know ..handy information!!