POLICE need the public to help them as they investigate a robbery on Tuesday, 5 January at around 7.30pm.
The boys were followed to Chafford Hundred station by four youths as they returned from a shopping trip at Lakeside.
Detective Constable Sally Everritt said: "The robbers tracked their victims to a station shelter, where one of them held a knife to the victim and demanded cash.
"Fearing for his safety, the victim handed over his wallet containing Â£500.
"While this was happening, another member of the gang pushed one of the other boys against a wall, banging his head."
Images from the station have been distributed on police intelligence networks, and detectives continue to investigate.
DC Everritt said: "I would like to reassure the public that violent crime is rare on the rail network, but at the same time appeal for their assistance.
"This was a frightening experience for the boys involved and we need you to help us find those responsible.
"I believe the people in the images we are issuing today will be able to provide us with information which could assist our investigation. Please let us know if you recognise any of them."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference 142 of 26/2/16. Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.