Did Repair Work Cause Metro Crash?

Did repair work cause the accident on the D.C. metro on June 22 that killed nine people? The Washington Post reports that “Metro personnel replaced a signaling component in the tracks five days before last week's deadly Red Line train crash, federal investigators said today. After that work was done, the control system circuitry that is designed to prevent crashes did not perform properly, investigators said.” The track circuit in the crash area "periodically lost its ability to detect trains after June 17,” according to the National Transportation Safety Board.