Two boys who boarded a train at Earlswood were left injured when they were attacked at East Croydon by men who shouted abuse at them and took one of their wallets.
British Transport Police (BTP) is appealing for help to track down the attackers, who detectives said carried out the “nasty and wholly unwarranted assault.”
The boys had been among a group of teenagers who got on the train at Earlswood on Friday, January 31.
The violent assault took place at 11.50pm.
Det Con Helen Magill, who is leading the investigation, said: “A group of five teenagers boarded the service at Earlswood station and sat in the third carriage from the front in a six-seat configuration.
“A separate group of three people then sat near the first group after boarding at Purley.”
Det Con Magill, who is based at BTP Croydon, said: “One of the men in the second group then approached the teenagers and started shouting abuse at them.
“He approached one of the group, a 16-year-old boy, and punched him twice.”
She continued: “To try and stop the man, and responding to his threats, one of the group handed over his wallet. “The man then punched another 16-year-old boy from the group.”
She said: “The suspect and a second man then tried to block the group of five from leaving the train at East Croydon.”
The first victim received a black eye and a swollen lip, while the second boy assaulted had a bruised face.
Det Con Magill added: “This was a nasty and wholly unwarranted assault, and our officers have undertaken a number of actions to establish what took place.
“We are now asking for any witnesses who saw what happened to come forward, so we are able to establish the circumstances.
“Violence of any kind on the railway network is completely unacceptable, and we will do everything in our power to identify the person responsible and deal with them accordingly.”
Anyone with information is urged to contact the British Transport Police on 0800 40 50 40, quoting background reference LSA/B2 of 18/02/2014. Information can also be passed to the independent charity Crimestoppers anonymously on 0800 555 111.