A teenager suffered life-threatening injuries and was placed in an induced coma after being punched on an Armadale line train on Sunday morning,
The 19-year-old is in a critical but stable condition in hospital after the altercation, which happened after he and friends boarded an Armadale-bound train in Perth around 6.30am.
Police say that during the journey, between the Cannington and Beckenham stations, the man and his friends got in a fight with another group of men on the train.
The teenager was punched to the right side of his head, before a friend intervened and helped him to his seat.
The man who hit him got off at Beckenham, while the victim and his friends stayed on board until Maddington station.
He became unsteady on disembarking and had to be helped to a seat by a friend. He was then rushed to Royal Perth Hospital.
The offender is described as being aged 18 to 20 years, dark skinned, approximately 187cm tall, with a slim build and brown hair. He was wearing a chequered yellow shirt.
Detectives would like to speak to anyone who was on the train at the time.