Officers have today released a CCTV image of a man who they think may have information about the offence shortly after 9pm on December 14.
The victim was coming up the escalator from Platform 15 when she noticed a man trying to go down the same escalator.
She pointed him in the direction of the correct escalator, and was then sexually assaulted, British Transport Police (BTP) said.
Anyone who recognises the man or who witnessed the offence is asked to contact BTP by texting 61016 or calling 0800 40 50 40.
Or people can call Crimestoppers anonymously on 0800 555 111.