Two boys approached a teen on a train from Purley to Clapham Junction, implying they had a knife, and demanded he hand over his phone at around 9 pm on October 9.
The teenager was forced to get off the train where he was also made to sign out of his iCloud account.
Detectives would like to speak to those in the CCTV image as they believe they may have information which could help with their investigation.
Anyone who recognises them is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 714 of 9 October.
You can also contact Crimestoppers anonymously on 0800 555 111.
