Why did three Metro cars derail during rush hour on Friday? Excessive heat.

On Saturday, the tenth day in a row with temperatures over 95 degrees in the D.C. area, the Washington Metropolitan Area Transit Authority released a statement saying that investigators think this summer's heatwave likely caused a "heat kink" in the tracks, which in turn caused the derailment:

Investigators have determined that a misalignment of the rails, known as a "heat kink," was a probable cause of yesterday's derailment near West Hyattsville Station. Three cars of an inbound Green Line train derailed at approximately 4:45 p.m. as the train entered a tunnel from an outdoor section of track. There were no injuries.

Heat kinks are short sections of misaligned track caused by the expansion of metal rails in extremely high temperatures and prolonged exposure to direct sunlight. When the expanding rail cannot be constrained by cross-ties and ballast, the rail may expand outward from the normal track alignment.

WMATA spokesperson Caroline Lukas told The Huffington Post that there is a speed restriction of 35 miles per hour in place for trains using tracks that are exposed to the sun, and that this restriction will last "at least for the duration of the weekend." Lukas also said WMATA has "increased track inspections" to look for any other heat kinks.

Also being affected by the heat: roads. WTOP reports that, according to officials, "buckled pavement" is "prevalent" in both Virginia and Maryland.

Gazette.net produced a video from inside the derailed train -- watch it below:

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