A man touched himself indecently while staring at a 15-year-old girl on the train from Dartford.
British Transport Police (BTP) detectives have released an image of someone they want to speak to in connection with the "frightening" incident which happened on May 23.
A 15-year-old girl and a man separately boarded a train at Dartford at around 4pm to go to Cannon Street.
He sat opposite the teenager and began touching himself inappropriately while staring at her.
Detective Constable Andy Parkinson said: “He continued this behaviour until Woolwich Arsenal station, where he got off the train before catching another back towards Dartford.
“Since the incident we believe the same man has been sighted at Erith station.
“This was a frightening experience for the girl and we need to find the man responsible.”
As part of their investigation, detectives have released a photo taken by the victim.
DC Parkinson added: “We treat all forms of unwanted sexual behaviour on the rail network seriously and are following several lines of enquiry in this case."
“I believe the man in this image has valuable information regarding this incident.
“I would urge anyone who recognises him to contact us.”
Reducing sexual assault on public transport
BTP has launched Project Guardian a project which aims to reduce sexual assault and unwanted sexual behaviour on public transport in London.
According to the TfL survey, around 15 per cent of females have experienced unwanted sexual behaviour on the network, with around 90 per cent not reporting it to police.
One of the most common reasons for not reporting it is because they didn’t think it was serious enough.
If you have any information on this incident call BTP on 0800 40 50 40, or text 61016, quoting reference SSUB/B4 of 03/07/2014.
Or call Crimestoppers anonymously on 0800 555 111.