WASHINGTON — Metro officials have not determined the cause of Monday’s rare train derailment but they have ruled out sabotage or terrorism, debris on the tracks, cold weather and human error on the part of the train operator.
Spokesman Ray Feldmann issued a two-paragraph statement about the investigation but refused to answer questions about the Blue Line derailment, which took place on top of a rail bridge north of the Reagan National Airport station.
Transit officials would not respond to questions about the condition and maintenance history of the train and the track where the derailment occurred. They also would not say whether they have made any changes to their operations or maintenance routines in light of the incident.
Outside experts who have studied track configurations focused on the fact that the Blue Line train passed through an interlocking just before the wheels on the last rail car climbed the track and lurched violently to the left, shearing off an iron safety railing and smashing third-track insulators for 1,500 feet. None of the train’s 60 passengers was injured.
Derailments are more likely to occur at an interlocking, where tracks cross and trains change direction, than along straight running rail, according to data collected by the Federal Railroad Administration.
That’s because interlockings involve many moving parts, such as switches, that are set to guide the direction of a train, said Michael Coltman, of the Volpe National Transportation Systems Center, which conducts research for the federal government.
“There are a number of built-in moving parts, which if they move in the wrong place at the wrong time, is not good,” Coltman said. Interlockings also involve gaps in rail, as opposed to the uninterrupted steel of regular track.
Coltman estimated that 20 to 25 percent of train derailments occur around interlockings.
“It’s important to have an understanding of exactly what happened in that derailment, so any adjustments can be made to prevent more problems,” said Allan Zarembski, a transportation consultant to the New York City subway, among other transit systems.
As part of a 1997 study of derailments at interlockings, also known as turnouts, Zarembski analyzed derailments involving the New York subway and the Bay Area Rapid Transit system in San Francisco.
Interlockings can push a train off a track even if both the train and the rail are maintained within a range of standards, he said.
“What makes it potentially very dangerous is the maintenance between the wheel and the rail,” Zarembski said. “You can have a situation where the track is marginally within standards and the wheel is marginally within standards but that combination can be enough to cause a derailment.”
That’s why the rail at interlockings must be maintained to the highest standards and demand extra scrutiny, he said. “The complexity of the turnout lends itself to a much more sophisticated inspection and requires more time, more expertise,” Zarembski said.
Track walkers inspect every piece of Metro track twice a week to look for signs of a defect. The stretch where the wheels jumped the track was last inspected the morning before the derailment. A more comprehensive track inspection, performed with equipment that uses ultrasound waves to detect rail weakness, is done every three months and was performed on the rail in question on Dec. 9.
Metro officials would not say whether they perform special inspections of rail interlockings or treat them differently than they do regular rail.