The woman was is said to have been followed from the platform at Highbury and Islington on March 23 at 2.30pm, before a man decided to sit next to her on the train heading to Hertford East.
During this time, he reportedly asked her personal questions before sexually assaulting her, the British Transport Police said.
Detectives are investigating the incident and an image has been shared of a man who police believe may have information that could help with their investigation.
Anyone with information is asked to contact British Transport Police by texting 61016, or by calling 0800 40 50 40 quoting reference 382 of 23 March.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.
