Andrew Busch of SEPTA said the woman apparently was waiting for the train on the Market-Frankford line at the Allegheny Station when she fell about 5:45 p.m. Saturday.
He said she fell at the same time an eastbound train was entering the station and there would have been no time for anyone to help her off the tracks before the train hit.
He said investigators do not suspect foul play but were still trying to determine why she fell and cause of death.
The woman's name has not been released.
Service on part of the line was interrupted for about two hours.
--------------The Associated Press contributed to this report.
