Assemblyman John Burzichelli, who is serving as spokesman for the town, said the bridge apparently buckled as it did during a derailment in 2009. The bridge was rebuilt in 2010.
The National Transportation Safety Board was sending investigators to the scene.
Residents of Paulsboro, West Deptford and East Greenwich Township were told to remain indoors early Friday before an all-clear was given.
The derailment occurred on a bridge over Mantua Creek, which empties into the Delaware River just across from Philadelphia International Airport.
Three major freight trains cross the bridge daily, serving refineries and other customers in this industrial area. Calls to Conrail were not returned.
State Senate President Stephen Sweeney, whose district includes Paulsboro, said he had been told that complaints had been made in recent weeks about noise coming from the bridge and that Conrail was looking into it. But he said he didn't have any details.
Authorities said at least four of the cars on the bridge contained vinyl chloride and a fifth was hauling ethanol.
They said the derailed cars would not be removed at least until Friday night and that the removal effort could take even longer.
Most vinyl chloride is gone from your body a day after you breathe or swallow it.
Breathing high levels of the gas can cause you to feel dizzy or sleepy. Breathing very high levels can cause you to pass out, and breathing extremely high levels can cause death.
Most of the vinyl chloride produced in the United States is used to make polyvinyl chloride (PVC), a material used to manufacture a variety of plastic and vinyl products including pipes, wire and cable coatings and packaging materials.