Constable Anton Perham said a carriage that was pulled by the society's TR35 diesel engine was partly derailed about 11am on Saturday, after stones were used to jam the points.
''One set of wheels of the guard van derailed when it went across the points.
''It was rather silly. If we find someone [in connection with the incident], I understand there is a charge under the Railways and Corridors Act, a specific charge that relates to interfering with railway tracks.''
The Oamaru Steam and Rail Society operates a regular Sunday scenic passenger service that runs around Oamaru Harbour, and society acting manager George King said the society's small diesel engine had just been taken out for a test when one set of bogeys on the guard van derailed as it was being driven back to the work sheds.
Mr King said no-one was in the carriage at the time, and it would not be known if the carriage had been damaged until it could be inspected.
He said although the train was only travelling at about 5kmh, it could have been dangerous, and even more so if the society's steam train had been on the track instead of the diesel at the time.
''That one is a lot heavier.''
The Sunday scenic service was unaffected by the loss of one carriage, he said.
