The alleged incident occurred at Beckton DLR station at around 3.30pm on November 12, 2025.
According to the British Transport Police (BTP), a 14-year-old boy was confronted by three boys.
His mobile phone was taken, and a knife was reported to have been used to threaten the victim.
Two boys, aged 15 and 16, have been arrested in connection and released on bail while enquiries continue.
Officers have released a picture of a teenager they believe could help with the investigation.
Anyone who recognises him is asked to contact BTP by texting 61016 or by calling 0800 40 50 40, quoting reference 491 of 12 November.
You can also contact Crimestoppers anonymously on 0800 555 111.
