Emergency personnel work at the scene after a CSX freight train derailed on Sunday.Cliff Owen / AP

A CSX freight train derailed in Washington, D.C., early Sunday, leaked a dangerous chemical, and closed down a Metro station.

Investigators are still trying to understand what caused the train to derail. A total of 16 cars went off the track, one of which carried 15,500 gallons of sodium hydroxide, a highly caustic chemical. The U.S. National Institutes of Health says this about it:

At room temperature, sodium hydroxide is a white crystalline odorless solid that absorbs moisture from the air. It is a manufactured substance. When dissolved in water or neutralized with acid it liberates substantial heat, which may be sufficient to ignite combustible materials. Sodium hydroxide is very corrosive. It is generally used as a solid or a 50% solution. Other common names include caustic soda and lye. Sodium hydroxide is used to manufacture soaps, rayon, paper, explosives, dyestuffs, and petroleum products. It is also used in processing cotton fabric, laundering and bleaching, metal cleaning and processing, oxide coating, electroplating, and electrolytic extracting. It is commonly present in commercial drain and oven cleaners.

About half of the sodium hydroxide spilled onto the ground near the crash site, about 3 miles from the White House. Two other chemicals also leaked from containers: calcium chloride and ethanol. The train was headed from Cumberland, Maryland, to Hamlet, North Carolina.

John Donnelly, the D.C. Fire and EMS chief, told the Washington Post the fumes from the sodium hydroxide shouldn’t cause anyone health problems. CSX is spending about $170 million to renovate its system around the D.C. area, but some neighborhoods have fought the projects because they fear the updates will allow CSX to run more dangerous chemicals or crude oil through their communities.

This isn’t the first CSX crash. In April 2014, another freight train, this one carrying crude oil, derailed in Lynchburg, Virginia, and caught fire, forcing hundreds of people to evacuate.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.