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Officers have today released CCTV images of a man they believe will have information about the incident, which took place at around 1.45pm on Sunday, 25 March 2012.
Investigating officer, PC Tony Farrell, said: "A group of around five men approached and then sat next to a female passenger who was waiting for a train at Exeter Central railway station.
"One of the men then stripped naked in front of the woman. The woman walked away from the group and boarded a train.
"It is understood the group of men then caught a train to Exeter St David's railway station before boarding a replacement bus service to Newton Abbot.
"The man's behaviour was completely unacceptable and, although unharmed, the victim was left extremely upset by his actions.
I would ask anyone who knows the man in these photographs to come forward and contact BTP in confidence."