A man travelling on a Greater Anglia train to Shenfield was seen performing an indecent act on himself earlier this month.
British Transport Police have now released CCTV images of a man they would like to speak to in connection with the incident.
It happened on the 2.35pm service from Liverpool Street to Shenfield on September 8.
Detective constable Sally Everritt said: “Witnesses on the train saw a man performing an indecent act on himself.
“He left at Shenfield and was seen entering a shop before walking towards Hunter Avenue.”
Detectives have spoken to people who witnessed the incident and viewed CCTV from the train and the station.
They have circulated images on police intelligence networks and are today appealing to the public for help.
DC Everritt said: “This was a disgusting act which upset those who witnessed it.
“We will not accept such behaviour on the rail network and I am glad this was reported to us.
“I would now ask the public to help with our investigation. I think the man in the images has information about the incident. Who is he?”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference ESUB/B4 of 30/09/2015. Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.
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