It is reported that the incident occurred between Kentish Town and Highgate stations on April 30 at around 5.40pm.
Officers have heard that a man stood close to a woman on board the train, before sexually assaulting her.
Police would like to speak to this man (Image: BTP) An image has been shared by the British Transport Police of a man who detectives would like to speak to, as they believe he may have information that could help with the investigation.
The man is seen with a beard, wearing a hood and using wired earphones.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 369 of 1 May.
Alternatively, contact Crimestoppers anonymously 0800 555 111.