Police are hunting a man who attacked a stranger onboard a train in Purley.
The victim, a 58-year-old man, suffered facial injuries in the assault, which followed an argument between the two passengers.
The attack happened on the 6.30pm London Bridge to Reigate service on April 1 while the train was at Purley station.
Police constable James Kirk, of British Transport Police, 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.
"This was an unacceptable assault which 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."
British Transport Police today released an image of a man wanted in connection to the attack after officers from the force's Croydon team analysed the train's CCTV footage.
Anyone with information can call police on 0800 40 50 40 or text 61016, quoting reference SSUB/B4 of 13/05/2014. Information can also be passed to the independent charity Crimestoppers on 0800 555 111.