Washington (CNN) -- Federal investigators trying to unravel why a freight train carrying hazardous chemicals derailed last month in New Jersey have ruled out speed as a contributing factor, according to a preliminary report released this week.
The southbound Conrail freight train consisting of two locomotives and 82 cars, seven of which derailed in Paulsboro, was traveling 7 mph at the time of the accident, according to the National Transportation Safety Board, which analyzed the locomotive event recorder data. The maximum authorized speed over the 160-foot bridge is 10 mph.
In releasing its preliminary report, the NTSB did not say what caused the accident just before 7 a.m. on November 30.
Three of the derailed tank cars contained vinyl chloride. One of them breached releasing approximately 180,000 pounds of vinyl chloride into Mantua Creek and the surrounding area, the NTSB report said. A "shelter in place" order was briefly issued for Paulsboro, and people were told to stay in their homes with the windows closed. Twelve square blocks near the scene were evacuated.
After the accident, 22 residents were treated and released at local hospitals. The train conductor was also treated and released.
Conrail officials tell the NTSB the initial damage estimate is $721,114, which does not include the removal of environmental pollution and contaminants.
The NTSB also is looking at the possible role the moveable swing bridge played in the incident. Monday's preliminary report did not provide any clues about what investigators found regarding the swivel bridge, which was capable of rotating 90 degrees on a central pivot to allow waterway traffic to pass.
A day before the derailment, railroad workers responded to a report of alignment problems with the structure, the NTSB said. Two supervisors from Conrail, which owns and operates the tracks and the bridge, spent a couple of hours inspecting and making adjustments.