National Transportation Safety Board Investigators are trying to figure out why the Paulsboro moveable bridge suddenly collapsed last November, causing a train derailment that sent a toxic chemical cloud into the air.

Derailed train at Mantua Creek in Paulsboro (US Coast Guard)

During a special hearing in Washington, the panel heard testimony from the Conrail engineer, who was on the train that fateful day.

Mark Mather told the panel for a month before the November 30th accident - on several occasions - the bridge would be lined up and properly locked, so the train could cross it.

But then, a message would come back that the bridge had failed to operate, "and all these problems that were happening with this Paulsboro moveable bridge seemed to be reoccurring more often since that Hurricane Sandy."

Mather said there were numerous reports of problems at the bridge - from him and other engineers, but Conrail officials testified all of the problem reports had been investigated and repairs had been made.

Mather added the day of the collapse, everything on the bridge had looked good, and the train had started to cross the span, but suddenly there was an emergency notification.

"And I looked into the side view mirror and I could see the A frame portion of the bridge listing from side to side until it finally goes over like a tree. When it fell over like a tree, it was almost instantaneous, and from the Mantua Creek I could see a fog cloud coming up, so I said, 'Oh my god, the bridge is collapsing'. "

He also told investigators he then sent an emergency message to Conrail and exited the train, running to Commerce Street.

"Because there's two schools in that area, and our thought right there was for the public safety. If we were afraid to be in that fog cloud, we knew we didn't want anybody else to go through it as well."

A second day of hearings is scheduled for today.