It happened around 5:44 p.m. on SEPTA's Broad Street Line.
Two employees were working on the northbound tracks when police say one of the workers fell backward and was struck by a train. He later died.
A second worker suffered minor injuries, officials said.
James Southworth, the investigator-in-charge with NTSB's railroad division says there was train travel on all tracks when the accident occurred.
At this time, it appears the workers may have been inspecting the tracks at the time of the accident.
Southworth says his investigation has already begun.
The NTSB says the employee who was killed had worked with SEPTA for the past 17 years. He has not been identified.