A woman walked off the edge of the platform at the Ellsworth Street stop on the Broad Street line at 1:18 p.m. Tuesday, and suffered serious injury after coming into contact with the third rail.
"She just walks off the platform, there's no explanation for that," SEPTA Police Chief Thomas Nestell III said.
Surveillance footage shows the woman lean far over the platform on the northbound side looking for the train, then a moment later, step over the edge, falling on to the third rail.
Several passengers alerted the SEPTA cashier upstairs, who called for help.
Track workers performing maintenance down the tunnel also heard a "crack" from the electrical line which powers subways being momentarily interrupted when the victim's body hit the third rail and rushed to help while radioing the SEPTA control center.
Paramedics were able to carry the woman out of the station on a stretcher by 1:32 p.m.
The woman was brought to Jefferson Hospital and listed in serious condition.