Local train service to Trenton and Chestnut Hill was suspended for a short time because of the blast, WCAU-TV reported.
Police Chief Inspector Joseph Sullivan told KYW-TV investigators at first suspected a bomb because the blast left a crater 2 feet deep on the bridge. But he said they determined the cause was a "thermic event," water melting from icicles on the bridge coming into contact with overhead electrical lines for the tracks underneath.
"This was a significant explosion. There were large chunks of concrete thrown from one side of the street to the other and for a very long distance in an Easterly direction on the bridge. So that gave every indication that someone had actually detonated a device," Sullivan said.
An Amtrak engineer told police explosions caused by melting ice are fairly common but one as large as this is rare.
Four people on the bridge, two on foot and two in a car, were treated for minor injuries.