( Larry Levine/WMATA )

The transit agency has pulled 16 rail cars from service as part of its investigation. Those rail cars have 34 hubs that are the same as the one involved in Tuesday’s incident.

The root cause for the friction ring coming off “has not been determined,” Metro officials said in a news release. The transit agency has also not determined the cause of the possible hub failure.

Metro’s chief spokesman Dan Stessel said the investigation is continuing, and Metro is looking at the brake systems in all of the 190 rail cars that are its 5000 series.

The 5000 series has been troubled. The rail cars, made by CAF Inc. of Spain, had a history of electrical, software and wheel problems.

On Tuesday, when the friction ring came off the Blue Line train, there was a spark and smoke. The ring, which is similar to a brake disc on a vehicle, landed between the electrified third rail and the running rails of the track, according to Metro officials.

Two Orange Line trains that were behind the Blue Line train heading in the direction of Vienna were also damaged.

The Blue Line train kept going and passengers disembarked at the Smithsonian station. The train was then taken out of service.

Roughly 300 passengers on the Orange Line train behind it were stranded when that train “became mechanically disabled after striking the friction ring,” according to Stessel.

Passengers were evacuated, and streets around the L’Enfant Plaza and Smithsonian stops were shut down as crews responded.

No major injuries were reported.

When smoke was reported on the tracks, the third rail power was shut off, which caused another Orange Line train to lose power; it had to glide to the L’Enfant Plaza stop where passengers off-loaded.

In a news release, Metro’s General Manager Richard Sarles praised D.C. Fire Department officials for a “safe and orderly evacuation in a reasonable time frame” of passengers who were stuck aboard the train.

But many riders complained that they didn’t know what was going on, and some put out on Twitter that they self-evacuated. Metro said it had no official reports of people letting themselves off the train.

Matt Bassett, chairman of the Tri-State Oversight Committee, which helps oversee Metro’s safety, said Wednesday that Metro has “made us aware that all vehicles that failed in yesterday’s incident have been removed from service and they’re doing inspection and testing on them. We’re comfortable with their immediate response. We’re working with them in partnership with them on their investigation to figure out exactly what went wrong and why.”

Sarles said Metro will also work to “improve customer communications to passengers aboard standing trains and to others before they arrive at the impacted stations.”

He said work will also be done to improve radio communications where underground stations make it hard for equipment to consistently work.

Stessel said “radio performance for Metro Transit Police was poor in the underground environment” during Tuesday’s incident. He said the radios on board the trains, “all worked fine.” But the police, handheld radios “performed poorly.”

The L’Enfant station in particular is known as a troubled spot for radio communications. Radio noise sometimes “creates interference of the signal,” Stessel said. “In certain locations they’re unable to send or receive messages and if they can, they’re unintelligible.”

He said Metro plans to try to test technology to boost signals underground to improve its radio communications.

He said the lack of communications with Metro Transit Police radios did not impact the evacuation because D.C. Fire Department had a “better experience than our police did” with their radios.

He said Metro Transit Police “resorted to using their cell phones to call in information from lower level platforms.”

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