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ALSN is a train control system meaning Continuous Automatic Train Signalling used widely on the main lines of the ex-Soviet states (Russian Federation, Ukraine, Belarus, Latvia, Lithuania, Estonia). It uses modulated pulses inducted into rails similar to the Italian RS4 Codici and American Pulse Code Cab Signaling. On high-speed lines the variant ALS-EN is used which takes advantage of a double phase difference modulation of the carrier wave. The name ALSN is composed of ALS, literally "Automatic Locomotive Signalling" and the variant designation N "Continuous Effect" . Other variants are built according to the same scheme like ALST for "Fixed Point Automatic Train Signalling" and ALSR for "Radio-Based Automatic Train Signalling" . The term ALS-ARS refers to "Automatic Train Signalling with Automatic Speed Regulation" used in subways which is a form of an automatic train control system.