The man boarded at Flinders St and sat next to the victim when a seat became available.
During the journey he sexually assaulted the victim, causing the subject of the man’s unwanted attention to move away.
The offender followed the victim off the train for a short time when both exited the train at Seaford.
The man is of Indian or Middle Eastern appearance, in his 20s, with a slim build and short dark hair.
He was wearing a grey zip-up hoodie with a red print on the front, blue jeans and dark coloured sneakers.
Police would not confirm whether the victim was male or female.
Anyone with information is urged to call Crime Stoppers 1800 333 000.
