Officers are releasing CCTV images in connection to a serious assault after a man was left with a broken jaw.
The incident took place at 10.20pm on Saturday, April 14.
The victim, a man, was on board a Northern rail service travelling between Leeds and Sheffield when he went to use the toilet facilities.
Whilst waiting in a queue, he spoke with three other men.
As the journey continued, the men became aggressive, punching the victim to head in an unprovoked manner.
The victim fell to the ground and was then kicked multiple times by the three men.
He sustained a broken jaw which required surgery.
It is believed the offenders left the service at either Moorthorpe, Thurnscoe, Goldthorpe or Bolton-on-Dearne station.
Officers would like to speak with the man in the CCTV images as he may have information which could help with the investigation.
If you recognise him or have any information which could help, please text British Transport Police on 61016 or call 0800 40 50 40 quoting ref 149 of 15 April.