Federal investigators may have found the culprit in Monday's Metro crash in Washington, D.C.: a faulty circuit. According to The Washington Post, the Metro relies on a system of circuits embedded along the tracks to keep trains properly spaced. When a train trips a circuit, it transmits a signal down the line that automatically sets speeds for the cars and stops a train from crashing into the one in front. Investigators tested six circuits near the crash site and found that one circuit did not work properly. Maintenance work was done on the circuit this month and last year and investigators will examine those records. Because the track at the site of the crash was curved, and the train was traveling at the top speed of 59 mph, the train operator may not have had time to stop despite evidence that she hit the emergency brakes.