The victim was attacked around 9am today in East London on the Docklands Light Railway.
Stratford International Station, where the attack took place, is just a stone's throw from Westfield shopping centre.
The station was extremely busy as commuters used the service for their morning journey to work.
British Transport Police said in a statement: "Shortly after 9am this morning, officers were called to Stratford International station after reports of a serious assault.