A row over spilled ketchup on the seats of a Bexleyheath train led to a 13-year-old boy being punched in the face.
The victim, from Bexleyheath, was on a train from London Bridge when he got into an argument with a man thought to be in his 40s.
British Transport Police PC Andy Thompson said: “During the journey he was grabbed around the throat and punched in the face by a man following a verbal altercation over some tomato ketchup on the train seats.
“The victim and the man both left the train at Bexleyheath.”
The suspect is described as a white man of medium build with receding black hair and a distinctive walk.
He is around 5ft 7in tall and was wearing a grey coloured suit, a tie and a knee length black overcoat at the time of the incident at around 6.40pm on February 19.
Anyone with any information about the assault should contact BTP on 0800 40 50 40, or text 61016, quoting LSA/B5 of 04/03/14.
Information can also be passed on to Crimestoppers, anonymously, on 0800 555 111.