PAULSBORO, N.J. (AP) -- Conrail crews had reports of signal problems at a New Jersey 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 the industrial town of 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 (bridge) 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.
The veteran two-person crew operating the CSX train Friday was familiar with the route and had run it the three previous nights. They had started their shift at 3 a.m. in Camden and were surprised to get a yellow signal when they approached the bridge at about 7 a.m. They used a keypad device, similar to a garage door opener, to try to get a green light, to no avail. The pair then stopped the train for several minutes, examined the tracks, and got permission from a dispatcher to proceed, Hersman said.
The two locomotives and five tanker cars made it across before the crew looked back to see the bridge "collapse" and a pileup of cars in the creek. The one that ruptured had been struck by another tanker, she said.
Recordings of various data so far support the crew's account, investigators said. However, authorities are not yet sure whether the bridge deck actually collapsed or shifted.
Residents who went to a hospital Friday were treated for respiratory problems and eye irritation. None of the injuries was believed to be serious.
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.
"It could be a sludge, or even a solid. That will slow down any pumping operation," Coast Guard Capt. Todd Wiemers said Sunday.
There's a risk of another chemical leak until that work is done. So officials plan to keep the evacuation order in place through Saturday in a 12-block area that includes 48 homes, a private elementary school, a Head Start center and a few small businesses.
Three public schools in other parts of town will remain open.
"We'd rather have the kids in a controlled environment," Paulsboro Fire Chief Alfonso Giampola said. "Should something go awry, we know where they are."
As of Sunday, about 100 people were staying in motels, while an unknown number are with friends and family, said state Assemblyman John Burzichelli, a former mayor of Paulsboro.
Once that job is complete, the other wrecked tankers will be removed by crane.
The bridge usually supports at least three major trains each day serving refineries and other customers in the area.
The NTSB plans to review how the bridge was rebuilt after the coal train derailed there in 2009. That wreck was blamed on a bridge misalignment.
Investigators also want to learn if the tidal surge or debris from Superstorm Sandy may have caused problems at the bridge, although the area was not among those hardest hit by the storm.