Teenagers travelling home from an under-18s disco were among victims robbed by a "steaming" gang on a train between West Sussex and London.
Several victims have told British Transport Police (BTP) they had cash and other valuables stolen between Crawley and East Croydon on 29 August.
One 15-year-old boy was punched in the face while his phone and jewellery were stolen by members of the gang.
BTP said some victims may not have come forward because of their ages.
Appeal to parents
A spokeswoman said victims who were on the way home from an under-18s club in the Crawley area may have been reluctant to mention the incident to their parents.
"We are appealing to anyone who may have been making a return journey to London after attending the club and witnessed anything to contact us," she said.
"If parents had children who attended an under-18s club on the night, we need to establish whether their child may also have fallen victim.
"They could hold vital information to help police."
The robberies, between 2300 BST and 2245 BST, are said to have been committed by a gang of between 10 and 15 Afro-Caribbean males.
Steaming is a slang phrase describing often violent attacks by gangs on individuals on trains or buses. .