British Transport Police (BTP) have released CCTV images of a man connected to the alleged sexual offence.
Just before 9pm on July 18, a man walked down the train’s aisle and sat on a seat near a person.
He then exposed himself and masturbated while looking at the victim, who then got up and moved away from the man, BTP said on Tuesday.
Officers believe the man in the CCTV images 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 619 of 18 July. Alternatively, you can call Crimestoppers on 0800 555 111.