British Transport Police (BTP) are calling on members of the public to identify this man following reports of the incident on a train between Ascot and Camberley at 10.30am on Friday, May 25.
As a passenger passed through the train she witnessed a man expose himself while staring at another woman, sat nearby, before touching himself indecently.
The man left the train at Camberley and the witness later reported the incident to police.
PC Robert Leamon, the investigating officer, said: "This man's actions were outrageous and completely unacceptable and we'll do everything we can to catch him.
"The CCTV is very clear and I'm certain someone will recognise this man. If you do and can tell us who he is, we want to hear from you."
Contact BTP on 0800 405040 quoting reference B4/LSA of 2 July, or Crimestoppers anonymously on 0800 555 111.