The ticket inspector had puncture wounds to his arm and a bruised leg
A train passenger refused to have his ticket checked and his dog - thought to be a Staffordshire terrier cross - then bit the inspector, police have said.
The 61-year-old inspector had puncture wounds and bruising after the incident on the 1217 GMT service from London Victoria to Littlehampton, West Sussex.
British Transport Police (BTP) said the passenger got off at Worthing and fled when the inspector followed him.
CCTV images were released on Wednesday after the incident on 14 January.
BTP Sgt John Dickinson said the inspector had made his way through the carriage and approached the man, who had a tan-coloured Staffordshire terrier cross with him, asking to check his ticket.
"The man refused and the member of rail staff was then bitten by the dog," he said.
The dog attacked after the man refused to show his ticket, police said
The man got off the train at Worthing railway station where the member of staff approached him "to show him bleeding puncture marks to his arm", he added.
But he said the suspect "fled the station through the car park" after what was "a serious offence".
Sgt Dickinson said the ticket inspector needed hospital treatment for bruising to his thigh and puncture wounds to his arm.
The passenger was described as aged between 18 and 20, of medium build, and about 5ft 7in.
He was wearing a black-hooded top, black jeans, and a black baseball cap, and had a "distinctive" black and red backpack.