A man wearing a penguin costume was attacked on a train by a group of men after being asked if he supported Tottenham Hotspur.
The 19-year-old was on a service between London Liverpool Street and Chelmsford when the attack happened.
The victim's father and a member of the public were also assaulted when they tried to intervene.
British Transport Police have released images of three men they want to speak to in connection with the assault.
The man had passed the group of six men to go to the toilet when he was asked if he supported Tottenham Hotspur, police said.
When he came out of the toilet, the group blocked his way back to his seat and became aggressive before attacking him.
It happened between 23:50 and 00:25 GMT on 21 December and continued on the platform of Chelmsford railway station.