Police are searching for a man who looked over the cubicle wall at a woman in the ladies toilets at Epsom station.
CCTV images have been released of a man officers would like to speak with in connection with the incident on December 10.
The investigation comes after a Polish woman in her thirties was startled to find a man peering over the cubicle wall in the station toilets just before 10pm.
PC Alexandra O’Leary said: “A short while later, the suspect followed the victim as she boarded her train towards London.
“Realising that the suspect had also boarded the train, she approached a man asking if she could sit near him.
“Police are keen to speak with this man, who helped the victim before he left the train at Raynes Park station.”
The suspect is believed to have left the train at Worcester Park.
PC O’Leary said: “No-one should face any form of invasion of privacy like this and we will do everything in our power to trace the person responsible and bring them to justice.
“We are urging the man who helped the victim to come forward as he may be able to help us with our investigation.”
Anyone with information or anyone who recognises the man should call British Transport Police on 0800 40 50 40 or call Crimestoppers anonymously on 0800 555 111.
Please quote log B3/LSA of 7/1/13.