Officers investigating an incident on a train between Hemel Hempstead and Berkhamsted are today releasing this image in connection.
At 9pm on Thursday, October 21, a man sat opposite a woman on the train and covered his groin with a folder.
He then began touching himself inappropriately in front of the victim.
Officers believe the man in the image may have information which could help their investigation.
If you recognise him, or have any information, please contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 9 of 25/10/21.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.