PAULSBORO, N.J. — A New Jersey bridge where a train derailed last week, releasing a hazardous chemical into the air, had a series of rail alignment problems leading up to the derailment, the National Transportation Safety Board's top official said Monday.

Some of the problems were reported the day before Friday's derailment.

NTSB chairman Deborah Hersman said her agency has a lot more work to do before determining the cause of the accident. "Nothing has been ruled out," Hersman said.

And one important part of the investigation – a thorough inspection of the bridge and the derailed cars – must wait until crews can remove all the hazardous vinyl chloride from the area.

That work, too, paused Monday when vinyl chloride detection in the air nearby twice reached an unsafe level of more than one part per million. Residents were told at 6 a.m. and again at 6 p.m. to shelter in place, an order that effectively shut down the town and forced a halt to cleanup work at the spill site.

The earlier warning, which lasted five hours, led to school being canceled and businesses being closed.

The NTSB has been interviewing witnesses and investigating records, including details of a 2009 derailment of a coal train on the bridge, which is blamed on a misalignment of tracks. Hersman also said the agency is looking into the 23 "trouble tickets" about problems with the bridge over the past year. Hersman said nine of the reported problems came between Oct. 27 and Nov. 29.

She said some of the problems were minor, such as debris on the bridge and burned-out lights. But she said two involved problems with the signals or alignment.

The bridge, across the Delaware River from Philadelphia International Airport, swings out to the side to make way for recreational boaters in Mantua Creek below. Hersman said that the default position from March through November is to be open to boats. To be able to cross the bridge, train operators are to punch in a radio code that's supposed to make it close.

She said that at 3:15 a.m. Thursday, a crew found that rails on the single-track bridge were about four inches out of alignment. She said it was only after several tries that the radio code finally worked so that the bridge's four sliding locking mechanisms were was fully in place and the signal turned green, allowing them to cross.

Hersman said Conrail officials spent about two hours that day working on the bridge.

After they left, she said, four trains crossed without problems.

The last one went over around 11:15 p.m. Hersman said there was no problem getting over the bridge, but the train crew received an automated voice message from the bridge a few minutes after crossing: "Bridge failed to operate," the voice said. She said that could mean that the bridge did not fully reopen after the train passed.

The next train to approach was the one that derailed.

Hersman said crew followed its proper protocol upon seeing a red signal, though she did not say whether that protocol should be changed. The engineer tried the radio code several times and the conductor looked at the bridge to see that it looked passable.

Only after that, she said, the engineer called a dispatcher and asked for and received permission to cross despite the red light.

The two locomotives and six cars crossed safely, but the next seven cars derailed.

NTSB spokesman Terry Williams said the agency is looking into whether the dispatcher who gave permission to cross was following railroad rules correctly.

Bob Comer, an Ohio-based investigator of train accidents and frequent critic of the U.S. railroad industry, said there were several problems with the way things played out in Paulsboro.

"The engineer and conductor are not qualified to determine whether the bridge is safe," he said "The fact that some dispatcher told them to cross, that is outright negligence."

A spokesman for Conrail, which owns and operates the train track, did not immediately return a call on Monday.

___

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