Officers are investigating a sexual assault on board a train from Chadwell Heath to Whitechapel on June 16 at around 8am.
A man sexually assaulted another man as they stood near the doors of the train.
Images have been released of a man who detectives believe may have information that could help with their investigation.
Anyone who recognises him is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference 688 of 16 June.
Information can also be given anonymously to Crimestoppers on 0800 555 111.
