Police are searching for an 'aggressive' man who had to be challenged by four train passengers, including an off-duty British Transport Police (BTP) officer.
He eventually punched one of the men in the head as they tried to calm him down.
See also: Passenger brawl over first class seat causes travel disruption
See also: Man jumps from van second before train hits
Officers from BTP have released CCTV images of a man they'd like to speak to in connection with the assault on board a train between Thornton Heath and Wandsworth Common.
Shortly after 9pm on Saturday, 23 April a man was walking through the carriage when he was subject to threatening and abusive language by a suspect male. Appalled by this behaviour, a second member of the public then intervened with assistance from an off-duty BTP officer who were likewise subjected to abuse.
When the man could not be calmed, a further member of the public intervened who was then verbally abused and punched in the head. The suspect male then left the train service at Clapham Junction.
Investigating officer PC Andy Thompson said: "This man was a particularly aggressive individual who senselessly attacked an innocent member of the public. It was an entirely unprovoked beating and it is important that this person is identified.
Anyone with any information is asked to contact police by calling 0800 40 50 40 or sending a text to 61016 quoting reference 260 of 27/05/2016. Alternatively, information can be passed anonymously to independent charity Crimestoppers on 0800 555 111.