Metro said Monday that it has determined that “mechanical fatigue” on the friction rings of two of its older rail cars caused one ring to crack and another to fall off earlier this year.

The rings are similar to a brake disc found on a car, and if a ring fails, it is considered a hazard.

The findings are expected to be presented Thursday at a meeting of the safety and security committee of Metro’s board.

Crews discovered a fractured brake part on a 3000 series rail car in January during an inspection at the Shady Grove rail yard, according to documents prepared for Thursday’s meeting. The train was not in service at the time, Metro chief spokesman Dan Stessel said.

On Jan. 6, a similar part fell off a 2000 rail car series train on an Orange Line train that was headed toward New Carrollton. The train was disabled near the Stadium-Armory station, and there were delays for more than two hours. No one was injured, and passengers were transferred to other trains.

A "friction ring," a part of the braking system on Metro’s 5000 series rail cars, fell off a rail car in January and struck the electrified third rail, causing a shutdown of a portion of the Blue and Orange lines. (COURTESY OF WMATA)

Wabtec, a longtime equipment supplier for Metro, made the parts on the two trains, officials said. The agency said “fatigue failures have occurred with the original Wabtec” brake parts “before the end of their intended life,” referring to both of the failures.

Metro is sending materials to a private metals testing laboratory in Delaware for analysis. The agency continues to investigate “the factors that led to the mechanical failures,” Stessel said. The review will look at track conditions, design and assembly of the brake hub, and the materials used to make it.

After the incidents, Metro said it would inspect 464 rail cars that have similar brakes in its 2000, 3000 and 4000 series.

Metro has roughly 1,100 rail cars in its fleet, and they have been delivered in batches designated with a series number over the past 30 years.

The transit agency has had problems with other series of rail cars.

On Dec. 20, a friction ring fell from a 5000 series rail car, damaged two more rail cars and shut down service along the downtown core of the Orange and Blue lines for hours.

Metro officials said in March that it had known for six years that some rail cars had brake parts that fail sooner than expected.

Officials said the authority had planned to replace the parts in 2009 but never did the work because it did not have the money.

Richard Sarles, Metro’s general manager, has said it “took a couple of years to go through the design analysis” of what happened, and then Metro employees were consumed by the investigations into the 2009 Red Line crash. The issue did not come up again until the incident in December, he said.

Metro inspected the brakes on all of its 5000 series rail cars and pulled 16 from service to be examined for possible “hub failure.”