Metro Transit Police officers secure the entrance to L’Enfant Plaza Station in Washington, Monday, Jan. 12, 2015, following an evacuation. AP Photo/Manuel Balce Ceneta

A preliminary cause has been identified, but many questions remain.

A tunnel on the D.C. metro filled with smoke Monday afternoon, killing one woman and sending more than 80 people to hospitals. The Washington Post reported it took about 40 minutes before firefighters could evacuate the trapped passengers from inside six subway cars.

By the time they did, the air had grown thick with smoke:

The preliminary cause of the incident, the National Transportation Safety Board said, was electrical arcing.

Arcing isn't an unusual phenomenon when it comes to industrial sites, transportation safety expert Carl Berkowitz told me. "It's two pieces of metal, and an electric charge goes through,” he said. “The current will go between where it is being transmitted to where it isn't being transmitted. If the two pieces are close enough, the electrons will jump. It usually doesn't cause a problem."

In fact, you can try this at home (though it’s advisable you don’t): Take two nails, send an electric current through one, and bring the other closer to the first nail. A burst of flame-like flash will appear between the heads as the electrons move from the first nail to the second.

With larger sources of discharge, though, these flashes can create a pressure wave called an "arc blast" powerful enough to injure anyone in the vicinity:

Trains generally see this phenomenon happen without incident—if you've ever seen sparks fly when a train switches rails, that's electrical arcing—but as the NTSB found, the malfunction that occurred Monday came from the line's third rail. The third rail provides the current to power the train through the train's "shoe," or metal contact block that picks up the electricity.

That only solves part of the problem: NTSB officials said the "electrical arcing event" occurred about 1,100 feet in front of the train. What caused enough smoke to stifle the tunnel, therefore, is what Berkowitz said is the main mystery behind the incident.

"That’s what I don’t understand," Berkowitz says. “The charge happened ahead of the train, and smoke filled the tunnel. So what was at the head of the train?" Some likely guesses so far: water, debris, or enough garbage to create an arc.

In the meantime, it’s too early to tell exactly what happened. Berkowitz said electrical arcing is a solid first step, but according to the NTSB, the investigation could take months to conduct.

This post originally appeared on The Atlantic.

About the Author

Most Popular

  1. Coronavirus

    The Post-Pandemic Urban Future Is Already Here

    The coronavirus crisis stands to dramatically reshape cities around the world. But the biggest revolutions in urban space may have begun before the pandemic.

  2. A pedestrian wearing a protective face mask walks past a boarded up building in San Francisco, California, U.S., on Tuesday, March 24, 2020. Governors from coast to coast Friday told Americans not to leave home except for dire circumstances and ordered nonessential business to shut their doors.
    Equity

    The Geography of Coronavirus

    What do we know so far about the types of places that are more susceptible to the spread of Covid-19? In the U.S., density is just the beginning of the story.

  3. Traffic-free Times Square in New York City
    Maps

    Mapping How Cities Are Reclaiming Street Space

    To help get essential workers around, cities are revising traffic patterns, suspending public transit fares, and making more room for bikes and pedestrians.

  4. Perspective

    Coronavirus Reveals Transit’s True Mission

    Now more than ever, public transportation is not just about ridership. Buses, trains, and subways make urban civilization possible.

  5. photo: A lone tourist in Barcelona, one of several global cities that have seen a massive crash in Airbnb bookings.
    Coronavirus

    Can Airbnb Survive Coronavirus?

    The short-term rental market is reeling from the coronavirus-driven tourism collapse. Can the industry’s dominant player stage a comeback after lockdowns lift?

×