The alleged assault took place on board the train approaching Stratford International station on Friday (August 22) at around 10pm.
According to the British Transport Police, it was a man who was assaulted.
Images have been released of three men who police believe could help with their investigation.
Anyone who recognises the people pictured or who has information about the alleged assault is asked to contact police by texting 61016 or by calling 0800 40 50 40, quoting reference 760 of 22 August.
Alternatively, you can contact Crimestoppers anonymously on 0800 555 111.
