The Automatic Train Protection sub-system uses coded track circuit technology originally supplied by Rochester, New York-based General Railway Signal when the line was constructed in the 1970s. It is a life-critical system that provides a continuous stream of information to the train regarding the maximum safe speed via the running rails. Speed commands are designed to ensure trains maintain a safe stopping distance from any obstruction and do not exceed the speed limit for any segment of track. The system stops trains in advance of any other train traffic, and before stop signals at interlockings. The cab signalling system relies on track circuits to detect track occupancy, and thus send the appropriate speed code. Although the ATP system was designed to be fail-safe, the track circuit control equipment on which it relies has been prone to parasitic oscillations which can cause the system intermittently to fail to detect the presence of a train. This resulted in the 2009 collision, after which Metro began to operate all trains manually. In September 2014, Metro began reintroducing its computerized control system after taking a number of measures to avoid accidents similar to the 2009 accident.