The freight train that derailed in Paulsboro was required by railroad rules to stay off a troublesome bridge until the span was examined by a qualified railroad employee.
The question facing crash investigators is whether a cursory check by the train's conductor satisfied that rule.
Some rail experts say the train dispatcher should have waited for a track maintenance employee or signal worker to check the bridge after the train crew confronted a red signal Friday.
Instead, the conductor walked the bridge and reported to the train's engineer that it appeared secure. The engineer then requested, and received, permission from a dispatcher in Mount Laurel to cross.
The bridge gave way, dumping several tank cars into Mantua Creek. One ruptured, spewing hazardous vinyl chloride into the atmosphere and forcing the relocation of more than 400 nearby residents.
The evacuees, and those from 100 more homes added this week, were still displaced Wednesday, waiting for the derailed cars to be removed. Coast Guard officials overseeing the cleanup have said they may not be allowed back until Sunday.
Conrail, which owns the bridge and operated the train, has a rule that states: "When a train encounters a stop signal at a movable bridge, the train dispatcher must not authorize the train to pass the stop signal until a qualified employee examines the bridge and determines that the rails are properly lined and the bridge is safe for movement."
The swing bridge had an intricate locking mechanism to align its tracks with those on shore and to secure the bridge to its abutment. Four locks had to be in place for electronic sensors to trigger a green signal.
On Friday, the engineer used an electronic keypad to try to get the appropriate green signal, but could not. After the conductor walked the bridge and reported that it looked secure, the engineer received permission to proceed.
According to Conrail employees, that procedure has been used before, and trains have crossed bridges without incident. Employees of other railroads also said an engineer or conductor would be a "qualified employee."
But a former Conrail engineering executive said Wednesday that a qualified employee should have been a member of the railroad's maintenance-of-way department.
Conrail spokesman Michael C. Hotra declined to answer questions on the issue, saying the company would "defer comment" to the National Transportation Safety Board, which is investigating the crash.
An NTSB spokesman did not respond to an inquiry about the "qualified employee" rule.
A spokesman for the Federal Railroad Administration, Kevin Thompson, said the agency would not comment specifically on the investigations, but noted that railroaders performing safety-sensitive jobs are required by federal regulation to be trained and certified.
"The qualifications, training, and experience of the crew and the train dispatcher will all likely be part of the analysis," Thompson said.
Officials for Conrail, a subsidiary of CSX Corp. and Norfolk Southern Corp., knew before the wreck that there were problems with the Paulsboro bridge's mechanism and signal.
The bridge failed in 2009, causing the derailment of nine coal cars. And in the last year, Conrail received 23 "trouble tickets" from crews or others about the span, including nine reports since Oct. 27, NTSB Chairwoman Deborah Hersman said Monday.
A crew report on Nov. 19 indicated the bridge had not locked properly, and the day before the derailment, a crew reported the rails were four inches away from being locked, Hersman said.
Conrail supervisors made adjustments to the bridge mechanism last Thursday, and four trains crossed safely.
The last crew across, about 11 p.m. that day, reported that it had received a "bridge failed to operate" recording, indicating the span had not automatically opened as it should have when the train reached the other side.
About eight hours later, the bridge gave way under the tank cars on the train that proceeded through the red signal.