At around 1.10pm on June 27, a girl boarded at Tottenham Court Road when she sat opposite a man.
About 20 minutes into the journey he exposed himself and masturbate in front of her until the train arrived at Slough.
The man is then believed to have boarded a second train to Windsor and Eton.
Detectives would like to speak to the man in the image who they believe may have information that could help their investigation.
Anyone who recognises the man is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 371 of 27 June.
Alternatively you can call Crimestoppers anonymously on 0800 555 111.
