A group of Milton youths were caught red-handed after one of them threw stones at a train passing through the town on Saturday.
Constable Tom Taylor, of Balclutha, said the train driver had a video camera on board at the time of the incident.
One of the group smashed a train window, but he and his friends then became the unwitting stars of video footage shot by the driver.
This was later given to police and revealed the young people involved, who were spoken to soon afterwards, Const Taylor said.
Milton police believe a group of young people is responsible for a spate of wilful damage around the town.
Anyone with any information is urged to contact the Milton police on (03) 417 8203.
