It happened around 5:00 Friday evening under a bridge over railroad tracks on the 3100 block of Cecil. B. Moore Avenue.
"This was a significant explosion. Large chunks of concrete thrown from one side of the street to the other," said Chief Inspector Joe Sullivan, Philadelphia Police.
The large crater the explosion left behind had the bomb squad convinced that a live device had detonated.
However a bomb sniffing dog was brought in and nothing was detected. A second dog confirmed that there were no explosives on the scene.
An Amtrak engineer determined the explosion to be a freak accident.
Police say an icicle on the bridge came in contact with Amtrak high power lines beneath the bridge.
"That caused an explosion and an electrical arch which caused burning of debris. There was a pressure wave that came up through the bridge and created a debris field," said Inspector Sullivan.
Police say it was powerful blast that left behind a hole about two feet wide and a foot and a half deep.
An SUV crossing the bridge at the time was hit with debris. Two people, inside the vehicle, were rattled and transported to the hospital to get checked out.
One passenger described the shock of it all.
"The street just erupted. There was a hole in the street while we were driving. Like four to five feet in front of us. In a couple more seconds we would have been in front of the hole," said Hanif Styles.
Two pedestrians were also transported to St. Joseph's Hospital with minor injuries.