British Transport Police detectives are investigating after a man touched a woman inappropriately on a train as it left Waterloo Station on Friday 12 July.
Officers have released a CCTV image of a man they would like to speak to after the incident, which took place between 8.35pm and 8.50pm on a train travelling from Waterloo Station to Weymouth in Dorset.
"As the train left the station, a man approached the 45-year-old victim and asked to sit next to her," said Detective Constable Andy Parkinson.
"He offered to share his fast food meal with her, before touching her inappropriately and leaving her understandably distressed.
"The man, who was seen to be wearing a wedding ring, left the 8.35pm London Waterloo to Weymouth service at Woking station."
Officers are following several lines inquiry into the incident, including reviewing CCTV footage and have obtained an image of a man they believe can assist their investigation.
"This was a very intimidating experience for the victim and we will do everything in our power to catch the man responsible," said DC Parkinson.
"If you recognise the man pictured, or if you are the man himself, please come forward and speak to us. You may have vital information to assist our investigation."
Anyone with information should contact British Transport Police on 0800 40 50 40 or text 61016 quoting reference B12//LSA of 08/08/2013. Alternatively, information can be passed to the independent charity Crimestoppers, anonymously on 0800 555 111