12 A formal investigation by LUL and Infraco SSL in October 2003 found the cause of the derailment to be a broken rail. The track was inspected the night before, but the damaged rail had not been detected. The report notes that neither visual inspections by patrols, nor the current method of ultrasonic testing (which was undertaken), would have found, and will not reliably find this type of rail defect, which initiated on the underside of the rail.