The incident happened at Johnstone train station on Thursday, March 27, between 8.45pm and 9.15pm.
British Transport Police (BTP) say the man was seen to enter the station and make his way to the waiting shelter of platform two where he exposed himself and committed a sexual act.
He is described as being aged between 18 and 25 years, of thin build and having sharp facial features.
He was wearing a black woollen hat, a black bomber style jacket, grey tracksuit top and trousers and blue training shoes.
Detective Constable Lorna White is anxious to speak with anyone who was at Johnstone train station at the time or who was on a train which arrived and left the station during the times stated.
She said: "This sort of behaviour is totally unacceptable and I am appealing for anyone with information which could assist our enquiries in identifying the man believed to be responsible to get in touch."
British Transport Police can be contacted on 0800 40 50 40 or information can be given to the independent charity Crimestoppers on 0800 555 111.