The male conductor, 39, was attacked as the 12:30 Edinburgh Waverley to Aberdeen service approached its destination at about 14:55 on Thursday.
The attacker had boarded coach C of the train at Stonehaven about 15 minutes earlier.
British Transport Police (BTP) have urged any witnesses to the incident to come forward.
The attacker was about 5ft 7in tall, wearing a light-coloured baseball cap, a light brown and green jacket, a dark jumper, dark trousers and dark blue trainers.
BTP said he had been under the influence of alcohol.
Constable Craig Allan, the 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."
CCTV footage from the train and stations will be reviewed as part of the inquiry.
Police are hunting a passenger who punched a train conductor in an unprovoked attack.