The National Transportation Safety Board has found that mechanical problems led to a subway derailment in Washington, DC earlier this year that injured 23 passengers.
The Board said that the probable cause of the derailment of the Washington Metropolitan Area Transit Authority (WMATA) train was a wheel climb on a car as it traversed a standard turnout near the Mt. Vernon Square station. The wheel climb was initiated by a rough wheel surface created during maintenance, the Board determined. The accident was also caused by the lack of quality control measures to ensure that wheel surfaces were smoothed during the maintenance procedure, the lack of a guard rail on the No. 8 turnout, and WMATA's failure to have an effective process to implement safety improvements identified following similar accidents and related research projects.
"We cannot emphasize enough how imperative it is that WMATA adopt and implement the recommendations issued today," said NTSB Chairman Mark V. Rosenker. "We want to ensure the safest travel possible for the thousands of metrorail riders who commute daily on this system."
On January 7, 2007, about 3:45 pm, northbound WMATA Greenline Metrorail train 504 derailed one car (the 5th of 6 cars) as it traversed a crossover from track 2 to track 1. About 80 passengers were on board at the time of the accident. Twenty-three passengers were transported to local hospitals for treatment and released.
The Board's report states that the wheels on the fifth car had been trued (truing restores the shape of a wheel and removes surface defects such as flat spots) at a WMATA maintenance facility 2 days prior to the derailment using a milling machine, and the car had been returned to train service the day of the accident. The surface of the wheel that climbed the rail displayed a "fish scale" appearance and had pronounced ridges. The Board noted that a rough wheel surface finish from wheel truing increases the probability of a wheel climb derailment. Running the cars through curved yard track and turnouts can polish newly trued wheel surfaces until the rough finish is smoothed over the entire tread surface; however, WMATA does not follow that practice. The Safety Board therefore concludes that WMATA's lack of measures to smooth wheel surfaces after truing increases the potential for a wheel climb derailment.
The investigation found irregularities in the dimensions of recently trued wheels at WMATA, including the wheel that derailed in this accident. Although the overall profile of the wheel was accurate, measurements indicated that more material was being removed during the truing operation than was necessary. It was noted in the investigation that the cutting device on the milling machine that was used on the wheel was found to be out of alignment. The Board found that although the misalignment of the milling machine used in the wheel truing operation did not contribute to the wheel climb, it does indicate inadequacies in WMATA's quality assurance process.
Findings from this accident investigation have identified several items that are common to previous derailments that WMATA has experienced. The previous derailments involved empty trains traveling at low speeds while traversing No. 8 turnouts or as the train maneuvered through a curve with a radius of less than 500 feet. These derailments also involved some cars that had recently been cycled through a WMATA maintenance facility to undergo a wheel-truing process. However, the Board noted that WMATA failed to address these issues that have been previously identified to prevent future derailments.
As a result of this finding, the Safety board recommended that WMATA implement quality assurance procedures to ensure accurate wheel truing, including the regular alignment and indexing of cutting heads on wheel milling machine, and establish a process, including a single point of responsibility, for timely evaluation and action on proposed safety improvements identified in previous investigations and research projects. The Board issued four other recommendations to WMATA dealing with maintenance policies.
The Board's full report will be available on the website in several weeks.