The altercation took place at around 9.30pm on Saturday, August 7, leaving a man from Worthing with injuries to his face.
PC Billy Burstow, the investigating officer, said: "A group of men and women were travelling home after a day spent in Brighton when they began arguing and a fight broke out.
"This resulted in one of the men, a 30-year-old from Worthing, being punched in the face, leaving him with cuts underneath one of his eyes and on one of his cheeks. He was also left with a bruised and swollen eye."
Police were called once the fight had broken out and two of the group, including the man who had been punched and a 19-year-old from Worthing, were arrested.
The men were both subsequently released on police bail pending further enquiries until Tuesday, August 31.
PC Burstow added: "We are currently making local enquiries into this incident and are working to build up a more complete picture of what exactly took place.
"We know that the train was fairly busy and I am keen to hear from anyone who was on board and who could assist our investigation."
If you saw the fight or have information that could assist the investigation, call BTP on 0800 40 50 40.
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