Police want to speak to passengers who may have witnessed an indecent exposure on a train in Beckenham.
The incident occurred at around 7.15pm last Saturday (June 21) and began at Beckenham Junction station.
A 28-year-old woman waiting for a London Victoria-bound train was approached by a man who exposed himself and began behaving inappropriately.
Detective Constable Sarah Garden, of British Transport Police, said: "The woman moved away and asked another woman on the platform to stay with her.
"The victim and the man both left the train at Brixton station."
Det Con Garden added: "We will not accept any type of unwanted sexual behaviour on the rail network and will do everything we can to bring those responsible to account.
"It was a Saturday night in Brixton and we are confident people will have seen what happened. We would particularly like to speak to the woman who accompanied the victim onto the train, and a man who spoke to police in Brixton."
Anyone with information is asked to contact British Transport Poice on 0800 40 50 40, or text 61016, quoting reference SSUB/B7 of 27/06/2014. Information can also be passed to crimestoppers anonymously on 0800 555 111.