It happened on August 28 at about 6.30pm, with the victim assaulted by a man on two occasions between Stratford and Leyton.
Police said the man then followed the victim off the train at Leyton and tried to engage him in sexual conversation.
The force has released an image of a man it wants to speak to as it appeals for witnesses to come forward.
Anyone with information is asked to call 0800 40 50 40, quoting reference 640 of August 28, 2019. Alternatively, call Crimestoppers anonymously on 0800 555 111.