The incident happened on the train while it was in the area of the 2500 block of Kensington Avenue.
Police say the suspect attacked the 17-year-old girl and then pushed her out of the train at Huntingdon Station.
The suspect is described as a male in his 30s, about 5'10" tall and 195 lbs.
He was last seen wearing a White T-shirt, blue jeans and a yellow shirt wrapped around his head.