Police are looking for a man who performed a 'revolting' sex act in front of a woman after getting on at Sutton.
The man exposed himself on the train from Sutton to St Albans and got off the train at Herne Hill.
The incident happened on December 17 at about 2.40pm. Police released CCTV images today to try and track down a man.
Officers have not said if the man is a suspect or a witness.
PC Mark Luker said: “This was a revolting act that left the victim feeling extremely intimidated and upset.
"Nobody travelling on the railway should have to put up with these disgusting actions and we’re determined to find the man responsible."
Call 0800 405040 or text 61016 if you can identify this man.
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