ALSN

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 (АЛС-ЕН) is used which takes advantage of a double phase difference modulation carrier frequency.

The name ALSN (АЛСН - автоматична локомотивна сигналізація непрерывного действия) is composed of ALS, literally "Automatic Locomotive Signalling" (АЛС - автоматична локомотивна сигналізація) and the variant designation N "Continuous Effect" (Н - Непрерывного действия). Other variants are built according to the same scheme like ALST (АЛСТ) for "Fixed Point Automatic Train Signalling" (автоматична локомотивна сигналізація точечную действия) and ALSR (АЛСР) for "Radio-Based Automatic Train Signalling" (автоматическая локомотивная сигнализация с использованием радиоканала). The term ALS-ARS (АЛС-АРС) refers to "Automatic Train Signalling with Automatic Speed Regulation" (автоматическая локомотивная сигнализация с автоматическим регулированием скорости) usually known in English as an automatic train operation system.

The system makes use of pulses at 0, 25, 50 and 75 ppm. Extensions in the ALS-ARS automatic train operation system encode with 75, 125, 175, 225, 275 ppm. The common carrier frequency is 50 Hz but in some areas a 25 Hz frequency is used. Those tracks are marked as ALS-25 (АЛС-25) lines .

Since the 1990s, the Russian Railroad Company has introduced the computerized successor system KLUB-U which requires ALSN / ALS-EN sensors for compatibility. In ERTMS the ALSN/ALS-EN systems are listed as ETCS Class-B systems.