A 27-year-old commuter was left "shocked and uncomfortable" when a men started touching himself in front of her on a London-bound train.
The woman had been sitting on a train from Bexleyheath to London Bridge when a man sat down in the aisle opposite her and performed the indecent act, police say.
Afterwards, he got off the train at London Bridge.
Police have now released a CCTV image of a man they want to trace over the incident, which happened at 11.15am on June 27.
Investigating officer PC Jay Cremin from British Transport Police said: “We would like to speak to this man in connection with an offence of outraging public decency.
“The incident left the victim feeling shocked and uncomfortable at what had happened, and this is totally unacceptable."
Anyone with information should call British Transport Police on 0800 40 50 40 or text 61016 quoting 297 of 31/08/2016.
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