The victim, aged in her 30s, was travelling between Mile End and Leytonstone, in east London, when a man touched her inappropriately, British Transport Police said.
The woman turned to face the man, who then began to speak to her daughter, detectives added.
The pair fled the train at Leytonstone but the suspect followed and continued to speak to them.
He is then said to have left the train and returned to Stratford where he boarded a DLR train towards Woolwich Arsenal after the attack around 6.20pm on February 21.
Officers have now released CCTV images of a man they wish to speak to in connection with the incident.
Anyone who recognises him should call 0800 40 50 40 or text 61016, quoting reference number 228 03/05/2017.