Police are seeking three men over an attack on a man wearing a top hat.
The attackers struck on a train from Catford Bridge to Charing Cross in London and also stole the eye-catching hat from the victim.
They had earlier been rebutted by the owner when they asked to try it on, in the evening of 28 February.
Violence broke out on board a train when there was "a disagreement", said British Transport Police.
CCTV has been issued in the hope of tracking down the men.
Officer Stephen Allen said: "I am keen to speak to the men in the CCTV images as they may hold vital information to my investigation and I urge them to come forward."