Police today appealed for help in tracing a fresh-faced teenager after a boy was mugged at knifepoint on a train.

The attacker is believed to have ridden up and down for six hours between central London and Essex before robbing his victim of an I-pod music player and making off at a remote country station.

Detectives are intrigued by the behaviour of the young crook, who they suspect may have run away from home.

The 14-year-old victim, who was badly shaken by the ordeal, had boarded the train at Dovercourt and was travelling towards Colchester when he was confronted by the mugger with a craft knife.

He identified the suspect from these CCTV images.