A man is said to have boarded the train at Shadwell station on March 4 between 9.15pm and 9.20pm, when he sat opposite a woman.
It is alleged h then sexually assaulted the woman, and a member of staff assisted her.
He then left the train at Limehouse station.
The British Transport Police has released an image of a man who may have information that could help with the investigation.
Anyone who recognises them is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference 763 of 4 March. Alternatively give information anonymously through Crimestoppers on 0800 555 111.
