The victim has been on a TfL Rail service between Stratford and Ilford when a man, whom she did not know, touched her inappropriately below the waist.
She tried to move away but the man followed her, continuing to be inappropriate.
It happened at around 5.25pm on Tuesday, 11 December.
An image has been released of the man police want to speak to.
The pair who are pixelated are not being sought by investigators.
In a statement, British Transport Police said: “Officers believe the man shown in this image may have information which could help them investigate.
“If you know who he is, contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 425 of 11/12/2018.
“Or ring Crimestoppers anonymously on 0800 555 111.”Tags: Stratford, Ilford, TfL Rail
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