On Thursday 1 May between 9am and 9.45am a woman in her early twenties was followed by a man off the District line train at Mile End to head East bound.
Despite the platform not being busy he stood extremely close to her before sexually assaulting her.
The man then followed the woman onto a bench to wait for a train, before following her onto a train, staring at her throughout.
He then followed her off the train at South Woodford, but the woman jumped back onto the train as the doors closed to get away from him.
Detectives believe the black man in the image may have information that could help with their investigation.
Anyone who recognises them is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference 394 of 1 May.
Information can also be given anonymously to Crimestoppers on 0800 555 111.
If you or anyone you know have been affected by the people highlighted in this website, then please report those individuals to the Police on 101 (999 if an emergency) or visit their online resources for further details of the options for reporting a crime. You can also make a report at Crimestoppers should you wish to be completely anonymous. There is help available on our support links page.