Commuters wait for shuttle buses at the Federal Center SW Metro station after service was suspended following a non-passenger train derailment on Aug. 6. (Amanda Voisard/For the Washington Post)

A Metro derailment that crippled large parts of the subway for hours Aug. 6 resulted from “a failure” of the transit agency’s “quality check process,” in which a technician disregarded a rail defect found by a track-inspection machine several weeks before the accident occurred, according to a report issued Friday.

“The report finds that the root cause of the derailment is failure of the rail fasteners that hold the rail in place, which resulted in the rail spreading wide enough for the wheel axle [of a train] to drop between the rails,” said the document, prepared by Metro.

The “wide gauge” defect had been detected nearly a month before the derailment of six-car train No. 412, which was not carrying passengers. However, the technician who was operating the track-inspection machine “erroneously” disregarded the machine’s warning of the defect, thinking it was “an anomaly,” the report said.

The train jumped the rails between the Federal Triangle and Smithsonian stations shortly after the subway’s 5 a.m. opening Aug. 6. The accident forced Metro to close the two stations, in the downtown Washington core, and halt service on stretches of the Orange, Blue and Silver lines for nine hours, leaving tens of thousands of riders scrambling for alternatives.

The technician and one of his supervisors have resigned from Metro, according to the transit agency, which publicly apologized for the derailment and said it issued $750,000 in fare refunds to 158,000 SmarTrip card­holders who were inconvenienced.

The report will be the subject of a special meeting of the Metro board’s safety committee on Thursday. The panel’s chairman, Michael Goldman, said the agency will have no extensive comment on the report until then.

“It is important to note that in this incident, critical information was lost due to human error and the absence of a process that allowed the error to go undetected,” Metro said in a statement. “This was not a case of a known safety defect condition being intentionally ignored. . . . Human error is one thing all transportation carriers have to manage.”

At the site of the derailment, several fasteners that hold the rails in place were “either loose or missing,” Metro’s interim general manager, Jack Requa, said after the derailment. The report said a “track geometry vehicle,” which rolls along the rails and inspects them using technology, detected the fastener problem July 9.

Sometimes the track geometry vehicle senses a problem that doesn’t exist or isn’t serious, according to the report. It said technicians are trained to tell the difference between false warnings and true defects that need immediate attention.

On July 9, the report said, the vehicle “identified the wide gauge defect” between the Smithsonian and Federal Triangle stations, along with “three other similar ‘Level Black’ track defects. A ‘Level Black’ defect is a defect that needs to be addressed immediately because the track is at risk of failure if operations continue.”

The three other defects were fixed. As for the rail-fastener problem at the derailment site, the technician “erroneously deleted the defect from his . . . report, believing it to be a routine anomaly, such as those he is trained to see and delete,” the report said. It said the technician’s mistake went unnoticed by supervisors.

“That such a serious error went undetected with no checks and balances in place reveals gaps in Metro’s safety policies and procedures,” Goldman said in a statement.

The track geometry vehicle’s primary job is to find defects within rails that aren’t visible to track inspectors who routinely walk the rails, looking for problems.

“Still under investigation by Metro’s Safety Department is why the wide gauge condition and the broken fasteners went undetected by Metro track walkers who inspected this area of track on multiple occasions between [the track geometry vehicle’s] run on July 9 and the August 6 derailment,” Metro said in its statement.

For a transit agency chronically beset by infrastructure problems and service disruptions this year — including a Jan. 12 electrical malfunction that filled a rail tunnel with smoke, killing one train rider and sickening scores of others — the derailment was another in a series of black eyes.

The problems have caused growing frustration on Capitol Hill, where members of the Washington area’s congressional delegation have repeatedly castigated Metro for its management and operational shortcomings.

“While the resignation of two employees directly involved in the inspection process offers some modicum of accountability, I hope the Metro board will not hesitate to take further disciplinary actions, if necessary, upon completion of this investigation,” Rep. Gerald E. Connolly (D-Va.) said Friday.

The derailment report said Metro has taken several actions since Aug. 6, including increased supervisory oversight of rail inspectors.

The transit agency’s top engineer ordered emergency track inspections across the system and enhanced inspections in the immediate area of the derailment. Metro also has imposed speed restrictions on all curves as part of an “aggressive campaign” to inspect curved sections of track throughout the system.

Although track geometry vehicles have been widely used in mainline and commuter railroads, Metro said it is one of a few rapid transit systems to own such a vehicle, which it acquired in 2012.

However, “equipment alone is not an assurance of safety,” the agency said. “Safety requires that equipment is used in an appropriate way to define and eliminate risks. This incident has highlighted the need for the proper focus by everyone involved in track maintenance and quality process.”