FRIENDSHIP HEIGHTS, Md. (WUSA9) -- A tiny Metro track defect caused a three hours of delays on the red line is Tuesday morning still something of a mystery to the agency.

A Metro spokesperson says steel rails crack, as they did Tuesday morning near Friendship Heights, after big changes in air temperature. Metro says it’s not sure if that happened in this case.

Around 7:30 Tuesday morning, Metro rail controllers got a computer alert for a cracked rail. Trains began single-tracking while workers began on a temporary repair. Metro stresses riders, frustrated by the slow ride, were not in danger.

“It was real bad,” said rider Chris Sanders as he returned from an appointment. “On a scale of one to ten, I would say it was a seven and a half or eight.”

Metro does not know the age of the rail that broke.