"I can't imagine not helping someone in that situation," said Knafelc, who is originally from Western Pennsylvania.
Train traffic was halted and firefighters removed the man from the tracks. He was taken to Temple University Hospital complaining of pain, SEPTA Transit Police Chief Thomas J. Nestel 3d said.
"He's a hero," Nestel said of Knafelc. "This is what Philadelphia is all about."
Knafelc said he heard the last train leaving as he arrived at the station, so he knew when he jumped off the platform that the next one would not be along for five or 10 minutes. He also said he was fairly certain he would not get hurt by the third rail and was not near it when he was on the tracks.
"I jumped down. He was in pain. He was in agony," Knafelc said, adding that he tried to keep the man's head and neck stable.
Nestel said there was no immediate explanation for why the man fell. On the video, he can be seen walking normally, straight off the platform.
Knafelc said he had finished a visit to Temple University and was going home when the man fell. He was at Temple because "I plan on going back to school in the future," he said.
Contact Robert Moran at 215-854-5983, firstname.lastname@example.org, or follow @RobertMoran215 on Twitter.