The incident happened as the service travelled between Wakefield Kirkgate and Barnsley at around 4.10pm on Monday, February 3.
A man is reported to have punched a train guard after being challenged for a ticket. He was then pushed off the train by passengers who came to the aid of the victim.
Police have released a CCTV image of a man they want to speak to you.
Anyone who knows him is asked to contact BTP by texting 61016 or by calling 0800 40 50 40 and quoting reference number 370 of 03/03/20.