A man who boarded a train at Haywards Heath was treated by paramedics after having a bottle smashed over his head.
The incident happened onboard a London to Brighton train, at around 10.20pm on Saturday, 31 January.
The victim, a 19-year-old man from East Grinstead, boarded the train with some friends at Haywards Heath.
The group got involved in a conversation with another group of men already on the train.
Detective Constable Nigel Craig, from British Transport Police, said: “One of the group already on the train, began chatting with the victim.
“While the conversation initially appeared good-natured, it soon turned sour when the man, without warning, smashed a bottle over the victim’s head.”
Both groups left the train at Brighton. While the victim sought assistance, his attacker left the station before police arrived.
DC Craig said: “What began as a good-humoured conversation ended in an act of mindless violence. The victim sustained a nasty wound to his head that required treatment by paramedics.
“I believe his attacker is from the Brighton area. I also believe there were other people on the train that night who know who he is.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, quoting reference SSUB/B6 of 02/03/2015.