The attack happened just after 7pm on Wednesday, 5 February, when a man boarded the train at Custom House station and sat next to the woman.
He then sexually assaulted her before following her off the train at Whitechapel station.
The suspect continued to trail the woman through the station and onto a London Overground train, leaving her deeply shaken.
British Transport Police have now released images of a man they want to speak to, believing he may have crucial information to assist their investigation.
Anyone who recognises him is urged to contact British Transport Police by texting 61016 or calling 0800 40 50 40, quoting reference 191 of 5 February.
Alternatively, information can be given anonymously via Crimestoppers on 0800 555 111.
