Spokeswoman Jerri Williams of the Southeastern Pennsylvania Transportation Authority says surveillance cameras show the 30-year-old woman entering the Lombard-South station two hours after service ended for the day.
Williams says the video shows the woman "inadvertently walking off the platform into the track area where she made contact with the third rail" at 2:19 a.m. Wednesday.
Just after 5 a.m. Wednesday, the woman was struck by a northbound train.
The city medical examiner will determine the exact cause of death, and Philadelphia police and SEPTA officials are investigating.
After the accident, shuttle buses were used to transport riders between stations on either side of the scene.