British Transport Police said the incident was directed towards a child on board a service from Bridgeton in Glasgow to Dalmuir, West Dunbartonshire, at around 4pm on March 20.
The force has released CCTV images of the man they believe may have information that can assist them with their investigation.
He is aged about 60, of heavy build, with short greyish hair and glasses.
He was wearing a blue adidas hooded top with three white stripes down the sleeves, a black shirt underneath, blue trousers and black trainers.
He was carrying a white shopping bag.
Anyone with information can contact British Transport Police by text on 61016 or by calling 0800 40 50 40.
If you have information which you would like to share anonymously you can do so by calling Crimestoppers on 0800 555 111.