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Investigators have determined that a misalignment of rail, known as a “heat kink,” was the probable cause of a July 6 derailment near the Washington Metropolitan Area Transit Authority’s (WMATA) West Hyattsville Station.

Three cars on a Green Line train derailed as it entered a tunnel from an outdoor section of track, WMATA officials said in a prepared statement. No injuries were reported.

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 can’t be constrained by crossties and ballast, the rail can expand outward from the normal alignment, WMATA officials said.

The agency restored service on the Green Line July 9 after crews repaired the 1,000-foot section of damaged track.