The National Transportation Safety Board's preliminary report on a Paulsboro train derailment that released 180,000 pounds of a hazardous material doesn't say what caused the accident.

cCews rig a rail car in preparation for it to be lifted with a 150-ton crane from the Mantua Creek (US Coast Guard)

In the report released Monday, the NTSB says a train was moving at 7 mph when it derailed on a swivel-style rail bridge overt Mantua Creek in Paulsboro on Nov. 30.

That is below the speed limit of 10 mph.

NTSB chairman Deborah Hersman has previously said the agency is also looking into the mechanisms that lock the bridge into place.

A tanker car ruptured in the accident, releasing vinyl chloride into the air, leading to the evacuation of more than 300 families and businesses.

The final derailed car was removed over the weekend.

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