Metro was designed to use an Automatic Train Control (ATC) system, which comprises three sub-systems. Automatic Train Protection (ATP) protects trains by separating them so as to avoid collision. Automatic Train Supervision (ATS) routes trains and assists in maintaining adherence to schedule. Together, these two sub-systems provide input to the train's cab signals and to a third sub-system, Automatic Train Operation (ATO), which can control the trains directly. Since a train collision in 2009 caused by an ATC malfunction, Metro did not use the ATC system following the accident until it was temporarily re-introduced on the Red Line in September 2014 as a test program.