A man allegedly exposed himself inappropiately to the teenager while staring at her on the journey at roughly 7pm on August 27.
The girl’s mother confronted the man, who then got up and walked away, British Transport Police said.
Officers investigating the exposure offence have released an image of a man they want to speak to in relation to this incident.
The man in the image may have information which could help the investigation, police said.
Anyone who recognises him, or has any information has been urged to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 548 of 27 August. Alternatively, call Crimestoppers anonymously on 0800 555 111.
It comes just weeks after detectives released the image of a man who flashed a 19-year-old woman and threatened to rape her before throwing a pint glass in a horror attack at a central London pub.
The incident happened at a pub in Bishops Bridge Road in Westminster just before 11pm on May 18.
Scotland Yard said the man verbally abused the victim before exposing himself. He then continued his barrage of verbal abuse and threatened to rape her. Before he left the pub he threw a pint glass, which hit the wall and shattered near the victim.
Meanwhile a man riding a green bike in has been linked to a string of flashing incidents involving lone women walking in Wembley parks.
The women were flashed in Monks Park Gardens, Gibbsons Recreation Ground, Bruce Road and King Edward VII park.
After releasing the man’s image, DC Lewis Jelley said: “We take violence against women very seriously and I would encourage anyone who recognises the man in the images to contact us. I would also encourage anyone who witnessed the incidents to speak to us if you haven’t already.”