A teenager was sexually assaulted while travelling on the train.
The 18-year-old man was heading home after boarding the train at Hastings late at night when he was inappropriately touched.
Detectives from British Transport Police (BTP) are investigating the incident, which occurred on the train between Hastings and St Leonards Warrior Square.
As part of their investigation, officers have released an image of a man they want to speak to in connection with the incident, which took place on Tuesday, 27 August.
Detective Constable Steve Best said: “The victim, an 18-year-old man from Bexhill, boarded a train at Hastings around 11.10pm, heading back home. During the journey, a man sat opposite the victim, engaging him in conversation. When the victim stood up the man touched the victim inappropriately.
“Understandably shaken, the victim then ran off the train at the next stop, St Leonards Warrior Square.”
Police have been following several lines of enquiry into the incident, which happened on August 27, including circulating an image on police intelligence systems, but to no avail.
Officers are now calling on the public to help identify the man pictured, as they believe he has key information about the incident.
DC Best said: “Unwanted inappropriate behaviour like this can be a very distressing experience for anyone. I would urge anyone who recognises the man pictured, or knows his whereabouts, to contact us.”
Anyone with information should contact British Transport Police on Freefone 0800 40 50 40 or text 61016 quoting reference B4/LSA of 10/10/2013. Alternatively, information can be passed to the independent charity, Crimestoppers, anonymously on 0800 555 111.