A gang of up to 20 youths, including a 13-year-old, boarded the Caterham-bound Southern service at Forest Hill, south-east London, on Tuesday evening.
They then stole cash and mobile phones and punched one passenger in the face.
"It is a concern that some people have used Halloween as an event to prey on innocent passengers and rob them," said a British Transport Police spokeswoman.
She said the gang boarded the train at Forest Hill, carried out the robberies, and jumped off before it set-off.
The arrested youths, aged between 13 and 18, were being held in custody at a police station in Lewisham.
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