The incident occurred at around 6.50am on March 8 after a woman boarded a stationary Northern line train at Edgware Underground Station in Station Road.
After boarding, a man is reported to have approached and assaulted her.
READ MORE: Road closures affecting streets in Harrow in May
British Transport Police (BTP) have shared a CCTV image of a man they believe may have information that could help with their investigation.
Anyone who recognises the man pictured or who has information regarding the incident is asked to contact BTP by texting 61016 or by calling 0800 40 50 40, quoting reference 482 of March 8.
Alternatively, Crimestoppers can be called anonymously on 0800 555 111.
