A serial masturbator exposed himself to lone women on trains while keeping his face covered.
Several women were approached by a man while travelling on services from London Bridge to Forest Hill via Clapham Junction over the New Year period.
BTP Detective Constable Anthony Steaman said: "We received a series of reports from women saying they had been approached by a male who had his head and face covered who sat opposite them and sexually exposed himself.
"On one occasion it would appear he struck several times on the one train service."
The incidents occurred from December 28 to January 2, between 5.30pm and 10pm.
The BTP believe there may be other women who were victims or passengers who witnessed this man in the act and are appealing for them to come forward.
One arrest has been made in relation to the incidents.
Anyone with information should contact the witness appeal line on 0207 391 5275, quoting background log B14 of 17/1/2008.
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