The teenager was attacked on a westbound District Line train between Elm Park and West Ham, police said.
After she was assaulted, her attacker got off at Canning Town.
British Transport Police released CCTV footage of a man they wish to speak to over the assault, which happened between 2.20pm and 2.30pm on October 1.
Now, they have revealed he is known to frequent Becontree station in Dagenham.
PC Ben Martin-Richmond branded the attack “unacceptable” and said it was “something nobody should have to experience”.
He added: “Our investigation is ongoing and enquiries so far have led us to believe that the offender frequents the Becontree area, at various times of day and night.
“I am appealing specifically to people in thelocal area and anyone who uses Becontree station, as they may well be able to help identify the offender.
“I would like to hear from anybody who recognises the man shown in the CCTV images - I am sure that somebody knows who he is and it is very important that we speak to him as he may have information which could help the investigation."
Anyone with any information about the incident should contact British Transport Police on 0800 40 50 40 or text 61016, quoting reference number 323 07/12/16, or call Crimestoppers anonymously on 0800 555 111.
