Metro officials still don’t know what caused an eight-foot section of rail to fracture and cause a derailment outside of Farragut North earlier this month, but they said the crack was not visible to the train operator and the rail did not break until the train was traveling over it.

At a meeting of the Metro board’s safety committee Thursday, Chief Safety Officer Patrick Lavin gave a recap of the events leading up to and following the Jan. 15 derailment. Lanvin said transit officials likely won’t know what caused the derailment until independent experts conclude their analysis on the broken segment of rail, which could take weeks.

However, he said officials found rust on the fractured rail — an indication that the crack may have been there before the derailment.

But Lavin said he is confident in the quality of the track inspections that took place leading up to the derailment, and that he personally reviewed the inspection reports from the walking inspections that took place in early January.

Ultrasonic testing on that stretch of the track took place in August 2017. Metro sent the results of those tests to outside experts for reexamination. Those experts concluded that Metro staff properly interpreted the results of the ultrasonic testing and there was no evidence of defects from the data that was collected by the ultrasonic machine.

A derailment of a Metro train outside Farragut North on the morning of Jan. 15. (Alan Devlin)

At Thursday’s board meeting, Lavin also defended the 90 minutes it took for passengers to be evacuated from the derailed train. Metro staff and first responders initially planned to pull a rescue train alongside the derailed one to collect the passengers. But they later abandoned the idea out of concern that restoring power to the fractured rail could pose a fire risk.

Board members said they remain concerned about the radio communication issues in the aftermath of the derailment. According to Metro’s official timeline, it took 20 minutes from the time of derailment until a Metro Transit Police officer was able to inform the Rail Operations Control Center that a derailment had occurred.

Some board members said Metro should expedite radio infrastructure upgrades in the underground sections of the system. But they also praised Metro for the evacuation process, saying that while the slow process was not ideal, it was acceptable in light of the fact that all the passengers were returned to the station safely.