A train passenger tried to strangle a man who was travelling with his young daughter between Sheffield and Doncaster.
Police investigating the assault, which happened on October 14, between 7.54am and 8.25am, wish to speak to the man pictured.
They believe he may hold information that could help with their enquiries.
British Transport Police said a male passenger had placed his hands around the victim's throat during the unprovoked attack.
Anyone with information is asked to call British Transport Police on 0800 405040 or to text 61016, quoting reference number 151 of October 14.