A 58-year-old-man suffered a black eye after being assaulted on a Reigate-bound train.
The incident happened at about 7pm at Purley station, on board a London Bridge to Reigate service.
Officers from British Police's Croydon team have released a CCTV image of a man they would like to speak to.
PC James Kirk said: "The victim was travelling alone and sat next to a man he did not know.
"As the train arrived at Purley station a disagreement between the two men took place which quickly led to a physical assault.
"It left the victim with a black eye and swelling to his face.
"Everyone has the right to travel on the railways without the fear of violence.
"I'd like to speak to anyone who saw the incident or who recognises the man pictured, as I think he may have crucial information."
