CCTV images have been released after a man wearing a dress exposed himself to passengers on a train travelling from Lincoln to Grimsby.
British Transport Police would like to speak to the man pictured in connection with the incident that occurred on July 14 at 7:30pm on the 18.35 Lincoln to Grimsby service.
A group of men boarded the train and began shouting and swearing.
One of the men, who was wearing a dress with no underwear, climbed onto the train seat and exposed his buttocks and genitals to a female and others on the carriage.
The man and the group with him got off the train at Grimsby but despite a search by police shortly afterwards, they could not be traced.
Police Constable Rachael Croft, the investigating officer said: "The man’s actions were completely unacceptable.
"What he may have perceived as fun was in fact very upsetting and distressing for the other passengers on the train.
"If you have any information about the incident, however small, we would really like to speak to you."
Anyone with information should contact British Transport Police on 0800 40 50 40 quoting reference NEA2 of 19 October 2012 or call Crimestoppers anonymously on 0800 555 111.