Police are investigating the report which allegedly took place on March 3 at around 9pm.
The man is said to have boarded the train at Chalfont & Latimer station and approached a woman, when he exposed himself.
He then left the train at Chorleywood station.
An image has been released of a man who police believe may have information which could help with their investigation.
Anyone who recognises him or who has information about the incident is asked to contact British Transport Police by texting 61016 or calling 0800 40 50 40, referencing 156 of 4 March.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.
