Witnesses are being encouraged to come forward after a woman was subjected to inappropriate sexual behaviour at Hassocks station.
A spokesman for British Transport Police (BTP) said the incident happened on September 5.
They said: “At approximately 4.10pm, a man approached a woman and asked her to perform a sex act on him at the station.
“The man, who walked away after the victim challenged him, was black, in his twenties, approximately 6ft and with black hair. He was wearing a black top and trousers.”
Detective Sergeant Margaret Storey added: “I would encourage anyone who has any information about this distressing incident to contact us at the earliest opportunity.
“No one should have to experience unwanted sexual behaviour of any kind on the rail network and we are making every effort to trace this man and speak to witnesses as part of our investigation.”
People who were at the railway station at the time or who saw anything can call BTP on 0800 40 50 40 or text 61016, quoting 378 of 5 September.