British Transport Police officers are appealing for witnesses to come forward after a ticket examiner was threatened and assaulted on train travelling between Aberdeen and Laurencekirk.
The incident happened on Saturday, April 23, at around 10.40pm.
The 35-year-old male member of staff was grabbed around the throat by a man in his mid 50s for no apparent reason as the train, the 22.27 Aberdeen to Dundee service, approached Portlethen station.
The suspect, who is described as being bald, heavy build and wearing a black jacket, black trousers, a blue shirt and a black scarf, got off at Laurencekirk just before 11pm.
Officers have reviewed CCTV footage and the man responsible is seen being dropped off by the driver of a silver estate car around 1.30pm the same day.
Constable Nicky Ritchie is appealing for anyone who was on the train or at the station and has information which can help the enquiry to contact British Transport Police on 0800 40 50 40 or by texting 61016.
Information can also be given to the independent charity Crimestoppers on 0800 555 111.