A man travelling on a train from Sheffield to Leeds was harassed by a group of men who then attacked him.
The victim was travelling on the 1.50pm Northern Rail service on Saturday, July 4 when he was targeted by five or six men who boarded the train at Barnsley.
The men were all aged between 20 and 35 years old and were wearing light green and navy polo shirts.
Investigating officer PC Fiona Poyner said: “The victim was sat at the back of the train on a folding seat when a group of five to six men, who had been drinking, boarded the service at Barnsley station at around 2.12pm.
“The victim was called names by the group before being hit on the back of the head by one of them.”
Anyone with information should call 0800 40 50 40.