The incident unfolded at West Footscray Train Station about 4.10pm on March 12.
Investigators believe the man approached a 41-year-old woman who was sitting at the station before he began inappropriately touching himself.
The man quickly boarded a Sunbury-bound train following the incident.
Investigators have released an image of a man they believe can assist with their enquiries.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000.
