Police are studying CCTV footage after a conductor was assaulted on a north-bound train.
The 39-year-old rail worker was attacked between Stonehaven and Aberdeen on Thursday around 2.55pm, as the Edinburgh Waverley to Aberdeen service approached its destination.
The suspect is believed to have boarded the train at Stonehaven.
British Transport Police said the conductor was punched in the stomach for no apparent reason.
Constable Craig Allan, the British Transport Police officer in charge of the inquiry, said: “Rail staff should be able to work in an environment free from the threat of encountering violence or anti-social behaviour.
“This attack is unacceptable and I am appealing for anyone who was on the train and may have witnessed the assault to come forward.”
He is described as 5ft 7in, and wearing a light coloured baseball cap, a light brown and green jacket, dark jumper, dark trousers and blue trainers.
Police are appealing for witnesses but no CCTV footage is being made available at this time.
