At around 3.45pm on December 20 three men were on board a train travelling from Birmingham New Street station.
During the journey, the train guard challenged the men over their fares.
When they left the train at Wakefield Westgate, one of the men became very aggressive and assaulted the guard by grabbing him and shaking him, a BTP spokesman said.
Officers believe the man shown in the CCTV images may have information which could help the investigation and would like to speak to him.
Anyone with information is asked to contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 324 of 20/12/2017. Alternatively, ring Crimestoppers anonymously on 0800 555 111