A teenager has been sexually assaulted on board a morning peak-hour train in the city's south-east, sparking a police probe to find the offender.
The teen got onto the city-bound train at Narre Warren at 7.35am last Thursday. Her attacker boarded the train at the same station.
Detectives from the transit crime unit said the man then assaulted the teenager by thrusting against her back and groping her.
"After boarding, he repeatedly pushed his groin into the back of the teenage woman and groped her buttocks," a police spokeswoman said.
"The woman tried to move away from the man in the crowded carriage, but he continued to follow her."
Police have released an image of a man they want to question in connection with the disturbing case.
Anyone with information is asked to contact Crime Stoppers on 1800 333 000.