The Jan. 12 Yellow Line incident damaged the electrified third rail near L'Enfant Plaza.

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The cause of Monday’s Metro incident that filled L’Enfant Plaza Station with smoke and and left one passenger dead is slightly clearer now, with the release of a preliminary report from the National Transportation Safety Board.

The report confirms that electrical arcing was at least partly responsible for the heavy smoke. Arcing occurs when an insulator on the electrified third rail gives off sparks or smoke. According to a 2013 Metro report, arcing occurs about twice a month.

According to NTSB’s report, investigators “observed severe electrical arcing damage to the third rail and electrical cables.” Damage from the arcing can be seen in the photo above.

An electrical breaker at one end of a section of the third rail tripped, or opened, at 3:06 p.m., according to the report. Ten minutes later, Metro activated ventilation fans to clear smoke from the area. But the breaker at the other end of the rail section remained closed, so power continued flowing to the damaged rail until Metro opened that breaker at 3:50 p.m.

Two Metro trains were affected. Train 302 stopped with its rear car 386 feet south of L’Enfant Plaza at 3:15 p.m. after encountering heavy smoke in the tunnel. The following train stopped at L’Enfant Plaza at L’Enfant Plaza at 3:25; passengers on board were also exposed to the smoke.

The Jan. 12 incident damaged the Yellow Line third rail near LEnfant Plaza.Enfant Plaza.

The brief preliminary report does not address one of the critical failures of the incident, the prolonged delay before rescuers appeared on the scene. NTSB continues to review Metro maintenance records, documentation on previous smoke events, and Metro’s emergency response plans, and have collected material samples from the site. NTSB is also interviewing personnel involved in the incident and has created eight investigative working groups.

Photos from NTSB