A ticket examiner was threatened and assaulted by a train passenger as he went about his work.
The incident happened on the 10.27pm Aberdeen to Dundee service on Saturday as it approached Portlethen station.
British Transport Police (BTP) said the 35-year-old staff member was grabbed around his throat by a man in his mid-50s “for no apparent reason”.
Officers are now appealing for witnesses to come forward and have issued a description of the suspect.
He is said to be bald, with a heavy build and was wearing a black jacket, black trousers, a blue shirt and a black scarf.
He got off the train at Laurencekirk just before 11.00pm.
BTP said officers have looked at CCTV pictures and the suspect is also seen being dropped off by the driver of a silver estate car at about 1.30pm the same day.
Constable Nicky Ritchie is appealing for anyone with information to contact BTP on 0800 40 50 40 or by texting 61016. Information can also be passed to Crimestoppers on 0800 555 111.