Images have been released of the man police are appealing to find (credit: BTP)
A man exposed and touched himself inappropriately in front of another passenger on-board the train, just after 2pm on Monday, 1 March.
The victim, who got on at Basildon, confronted him about his behaviour and he stopped.
After walking through the carriage, he then got off the train at West Horndon station.
Images have been released of the man police are appealing to find.
Anyone who recognises him, or with any information, please contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 2100012988.
Alternatively, you can call Crimestoppers on 0800 555 111.
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Tags: London, Essex, Trains, West Horndon, Basildon