Transit Crime Investigation Unit detectives are investigating a sexual assault in Huntingdale.
Investigators have been told the female victim was at the Huntingdale Railway Station about 12.30am on 3 March.
It is believed a man approached the woman as she sat alone on some steps outside the station.
He exposed himself and masturbated in front of the woman before walking onto the platform and boarding a Pakenham-bound train which investigators believe he may have exited at Noble Park or Dandenong.
Investigators have released images of a man who they believe can assist with their enquiries.
He is perceived to be Indian in appearance and was wearing a blue polo shirt, grey track pants and black shoes.
Anyone who recognises the man or has information regarding the incident is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au