The girl was in Walthamstow between 12.15am and 12.30am on Saturday when she met three men, police said.
She was taken to a garage area at the end of Hibbert Road, near Theydon Street, where she was pinned down and raped by two of the men.
The girl managed to call a friend who came to the scene and they left together. The suspects then left the area.
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The girl was travelling on a train from London Liverpool Street to Romford in Essex after going to see the poppies at the Tower Of London.