Police have released CCTV images of man after a ticket inspector was verbally abused and assaulted on a Peterborough to London train.
British Transport Police want to speak to the man in the footage about the attack, on the Peterborough to Kings Cross service on 18 September, at around 5.45 pm.
Investigators launched the appeal as part of a push to find four people wanted for similar assaults against rail staff.
Superintendent Phil Wilkinson said: “The officers investigating these cases have followed a number of extensive enquiries in order to identify those responsible, including circulating images on police intelligence systems, but to no avail so far.
“Now we are asking for members of the public to come forward and help officers in this investigation.
“If you know any of these people and can tell us who they are, then we want to hear from you.”
Anyone with information should call British Transport Police for free on 0800 405040 quoting B9/LNA of 27/02/2013, or call Crimestoppers, anonymously, on 0800 555 111.