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Investigators have released a CCTV image of a man they would like to speak to in connection with the incident, which took place between 7pm and 7.30pm on Sunday, 11 November 2012.
The 30-year-old victim was on board the 1910hrs Penzance - Exeter St Davids service when the incident took place.
Investigating officer, Detective Constable Dave Bailey, said: "The woman went to use the toilet in middle carriage of the train. As she passed a group of men who were sat in the carriage, one of the men approached the woman and touched her inappropriately.
The men left the service when it arrived at Truro station.
DC Bailey added: "The victim was left shocked and deeply upset by this attack.
"Incidents like this are very rare, but this was clearly a very distressing experience for the victim.
"We'll do everything in our power to catch the person responsible.
"Enquiries are ongoing and I would now ask the public to help identify the man in this photograph as I believe he will have key information which could assist this investigation."
Anyone with information about the man's identity, or his current whereabouts, is asked to contact British Transport Police on Freefone 0800 40 50 40 or the independent charity Crimestoppers anonymously on 0800 555 111.
In all calls, please quote incident reference B4/WWA of 12/12/2012.