Fail-safe In D.c. Metro Didn't Work

Malfunction In Key Circuit To Prevent Crashes Was Missed

July 02, 2009|By Lena H. Sun and Lyndsey Layton | Lena H. Sun and Lyndsey Layton,The Washington Post

WASHINGTON - -Five days before last week's deadly Red Line accident, a Metro crew replaced a key piece of equipment designed to prevent rail crashes, but the circuitry malfunctioned and no one in the subway system detected the problem, investigators and transit officials said yesterday.

The findings raise new questions about whether Metro officials should have discovered the hazard before one train rammed into another June 22, killing nine and injuring 80. It also puts a spotlight on the Metro's maintenance crews and the design of a highly automated subway system that is supposed to be fail-safe.

Transit officials would not say yesterday whether they believe the malfunction was a result of faulty equipment or poor installation, noting the ongoing investigation by the National Transportation Safety Board.

In the aftermath of the deadly crash on the Red Line between the Takoma and Fort Totten stations, Metro officials analyzed track circuit data and found that one circuit in the crash area intermittently lost its ability to detect a train. The circuit would report the presence of a train one moment, then a few seconds later, the train would "disappear," only to return again.

The problem started shortly after June 17, when a Metro crew replaced a device known as a Wee-Z bond, a crucial part of the system that maintains a safe distance between trains, said Dave Kubicek, the Metro's rail chief.

Instead of failing completely, the track circuit "fluttered" on and off so quickly that Kubicek said the failure would not have been obvious in the Metro's downtown operations center, where controllers monitor real-time movement of trains by watching an illuminated graphic depiction of the 106-mile subway system.

"It was happening so fast, you would just blink and miss it," he said. "Realistically, you had to be looking at the exact area at the exact place at the exact time."

A controller would have had to be staring at something the size of "a button on a BlackBerry" to detect the malfunction, he said, adding that the Metro did not notice the problem until officials began examining data in the aftermath of the accident.

Under normal conditions, if a track circuit goes "dark" or stops working, downtown controllers will see an indicator change colors on the illuminated screen before them. In addition, if the circuit stops working, the adjacent track circuits will automatically force approaching trains to stop before they reach the "dark" stretch. As a backup, the controllers can also intervene to redirect train movement.

But the "fluttering" was so fast and subtle that none of those auxiliary safety measures deployed, Kubicek said. He stressed that the malfunction was an "anomaly" and that the Metro rail system is safe.

He said the Metro is testing daily to determine whether this "fluttering" is taking place elsewhere. Metro workers have been inspecting all of the subway system's nearly 3,000 track circuits. They have checked more than 65 percent of them and found no other problems, Kubicek said.

"From what we have discovered so far, it appears to be a freak occurrence," said John B. Catoe Jr., the Metro's general manager.

The Frederick News-Post first reported the Wee-Z bond problem June 26.

Metro officials would not say whether in the days before the crash, other trains experienced trouble along the stretch affected by the malfunctioning circuitry.

NTSB officials said they are reviewing the performance of the track circuit before and after the equipment was replaced June 17.

The Metro rail system is divided into blocks and is designed to keep at least two blocks of distance between trains to prevent a crash. Each block contains at least one track circuit that detects the presence of a train, using audio frequencies transmitted between the train and the steel rails. Within each track circuit are two devices, the Wee-Z bonds, which are about 18 inches square and 6 inches high and are often mounted on the wooden cross ties that secure the rails. The Wee-Z bonds 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 Wee-Z bond sends a "zero" speed signal that forces that train to stop.

After the installation, Metro crews tested the equipment.

"Everything tested OK upon installation," Kubicek said.

The Metro is replacing Wee-Z Bonds throughout the rail system because many are approaching the end of their useful lives, said David Couch, who is in charge of the Metro's infrastructure projects.

In last week's crash, the train heading toward Fort Totten slammed into the train in front of it, which had been idling on the Red Line outside the station, waiting for a third train to depart.

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