A 52 year old man from Woldingham suffered cuts and bruises in an affray on a Victoria to East Grinstead Train on January 26th.
He boarded the 9.23pm East Grinstead-bound train at Victoria, and sat in a window seat.
Another man got on the train at East Croydon and sat down next to him.
A short time later, the two men got into a verbal disagreement, which turned violent.
The attack was only halted when other passengers intervened.
British Transport Police have released an image of a man they want to speak to following the incident between 9:30 and 10pm.
Update (Sun 15 Feb) : Police have told us that a man has now come forward.