She saw a man exposing himself through the window and the same man followed her as she left the station, according to officers.
She walked south on Pharmacy Avenue, near Teesdale Place. The man approached her from behind and sexually assaulted her, police said.
The man ran away on foot northbound.
The suspect is described as a brown man with a thin build and short dark hair. He is around five-foot-eight and five-foot-ten and is 18 to 25 years old. He was unshaven, police said. He was last seen wearing black trackpants with a white stripe down the leg, a grey jacket and a black backpack.
Anyone with information can contact police at 416-808-7474.