A Perth man is fighting for his life after he was punched in the side of his head while riding on the Armadale line on Sunday morning.
Kensington detectives said the 19-year-old man was with friends when he boarded an Armadale-line train at Perth station at about 6.30am on January 21.
Between the Cannington and Beckenham stations, the man and his friends became involved in a fight with another group on the train.
The 19-year-old was punched to the right side of his head.
The group who assaulted the man got off at the Beckenham train station and the victim and his friends stayed on the train until Maddington.
The man became unsteady after the punch and had to be helped by a friend to a seat on the platform.
He was taken by ambulance to Royal Perth Hospital where he underwent emergency surgery for a critical brain injury and was placed in an induced coma.
He remains in a critical but stable condition.
The offender is described as being aged 18 to 20, dark skinned, around about 187 centimetres tall, with a slim build and brown hair.
He was wearing a chequered yellow shirt.
There were several people on the train at the time of the incident and detectives would like to speak to anyone who saw the fight.
Anyone with any information regarding this incident is asked to call Crime Stoppers on 1800 333 000, or make a report online.