The alleged incident occurred on a service bound for Tottenham Court Road from Slough at around 9am on November 11.
A woman was standing in a busy carriage when she was reportedly sexually assaulted by a man who had been stood behind her.
According to the British Transport Police, the assault continued as she tried to get away.
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Officers have now released the picture of a man they wish to speak to, who they believe could have information that would help with the investigation.
Anyone who recognises him should text BTP on 61016 or call 0800 40 50 40, using the reference 201 of 11 November.
Information can be given anonymously to Crimestoppers on 0800 555 111.
