Six Critical Questions About Metro's Red Line Crash
Saturday, July 4, 2009
Twelve days after a deadly Metro crash on the Red Line, federal investigators are trying to answer several key questions that will help determine whether the cause was systemic failure in an aging railroad or a "freak occurrence," as Metro officials claim.
Although investigators have determined that a component designed to prevent crashes failed, they have not concluded what caused the June 22 accident that killed nine and injured 80 when a train rammed into an idling one north of the Fort Totten Station in Northeast Washington. The crash was the deadliest in Metro's 33-year history.
"The guiding principle of investigating accidents is that there's never one cause," said Elisa Nichols, a transit system safety consultant. Short of a meteor falling out of the sky and hitting the train, she said, the cause is always a combination of factors: "All the bad things line up at once."
Investigators from the National Transportation Safety Board are examining everything: the condition of the trains, tracks and signals; the actions of the operator of the striking train, Jeanice McMillan, who died in the crash; communication from her downtown supervisors; and the systems that control train movement and are supposed to automatically prevent crashes.
Among the critical questions that remain unanswered:
Is the Wee-Z bond to blame?
On June 17, a Metro crew replaced a device known as a Wee-Z bond, a crucial part of the track circuit system that maintains a safe distance between trains. The device, about 18 inches square and six inches high, is positioned between the running rails. It is a metal container that houses electrical and electronic components and is connected to the running rails by cables. Wee-Z bonds define a block, or distinct section of railroad. Each block contains at least one track circuit. Together with equipment in the rail car, the system is designed to prevent a train from entering a block occupied by another train. The system uses audio frequencies transmitted between the train and the steel rails to note the presence of a train and automatically transmit signals to the next train down the line. If the following train gets too close, the system sends a "zero" speed signal that forces the train to stop.
Shortly after the repair, the track circuit intermittently lost its ability to detect a train. After the crash, that same track circuit failed to detect a test train stopped in the same location as the idling train in the accident. The findings by the NTSB suggest that if the circuit was malfunctioning on the day of the crash, the system would not have detected the idling train and would have sent a "clear" signal to the striking train. Onboard computers would have set the train to 59 mph, the speed limit along that stretch, and into the path of the idling train.
The Wee-Z bonds are manufactured by Union Switch & Signal, Metro officials said. A spokesman for Ansaldo STS, the parent company of Union Switch & Signal, said he could not discuss the Wee-Z bond or the company's contract with Metro because of the ongoing investigation. Metro is replacing the devices throughout the railroad because many are approaching the end of their usefulness, officials said.
Investigators have stopped short of saying the malfunctioning circuit caused the crash.
Transit officials would not say whether they believe the malfunction in the track circuit was a result of faulty equipment or poor installation.
Is the work crew that made the repair to blame?