A train guard was abused and threatened while checking tickets by a passenger who boarded at Portslade Station.
The incident took place at 1pm on a Brighton to West Worthing train on Wednesday 17 July while it was between Portslade and Shoreham.
The British Transport Police issued CCTV pictures of the man they believe threatened the guard.
PC Pete Cooke, who is based in Brighton, said: “The member of staff was walking through the train checking passengers’ tickets.
“The man was then removed from the train at Shoreham and left the station.”
Officers want to speak to the man pictured or to anyone else who saw what happened.
A spokesman for the train operator Southern said: “This behaviour is completely unacceptable. Our member of staff was only doing his job.
“We would urge anyone who knows this man to come forward.”
Anyone with information is asked to contact the BTP on 0800 405040 or text 61016.
Information can also be passed to the independent charity Crimestoppers anonymously on 0800 555111.