The 24-year-old victim and a woman boarded a train at Leeds at 10.20pm on Saturday, August 5.
A British Transport Police spokesman said: "They sat next to two men and were chatting and joking, when suddenly one of the men punched him in the face. It left him with a black eye.
"The suspect got off the train at Adwick train station, whil the victim stayed on until Doncaster and then informed police."
Officers today released details of the assault along with an image of a man who they want to trace.
Anyone with information is asked to text 61016 or phone 0800 40 50 40, quoting 733 of 5 August.