CONRAIL CREWS had reports of signal problems at a Paulsboro, N.J., railroad bridge weeks before a train derailment Friday and were studying the problem the day before the crash and resulting chemical leak, federal investigators said Sunday.

Seventy people went to a hospital following the derailment. More than 100 people are expected to remain out of their homes this week while crews try to remove the hazardous material, vinyl chloride, from a ruptured tanker.

The National Transportation Safety Board cannot examine the scene until the chemicals are removed. But the agency this weekend began reviewing records with a focus on both the signal problems reported recently and a 2009 train derailment on the same bridge.

Conrail regularly moves tons of hazardous material over the low bridge, which was originally built in 1873. The bridge straddles Mantua Creek, a tributary near the Delaware River in Paulsboro. The bridge operates like a garden fence, with a section that swings sideways to open for boat traffic, then closes and locks into place to accommodate freight trains.

The NTSB will focus its probe on the locking mechanism and signal devices. The signals are triggered by sensors on the bridge, not by dispatchers.

"This is a very complex operation," NTSB chairman Deborah Hersman said at a news conference Sunday. "There is a lot of tonnage that goes over this bridge and a lot of hazardous materials."

Conrail crews in recent days and weeks had been reporting problems with the signal, and the rail company had been looking into the problem the day before, she said.

A team of NTSB investigators arrived in the region Saturday with scanners and other equipment to study the wreckage site, but they cannot get to the scene until the vinyl chloride is pumped out. The Coast Guard, Conrail and other agencies were coordinating that cleanup. But they were not sure how much of the gas in the ruptured tanker has liquefied, or how long it might take to remove.