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 of the carrier wave.

The name ALSN (АЛСН - автоматическая локомотивная сигнализация непрерывного действия) is composed of ALS, literally "Automatic Locomotive Signalling" (АЛС - автоматическая локомотивная сигнализация) 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" (АЛС точечного действия) and ALSR (АЛСР) for "Radio-Based Automatic Train Signalling" (АЛС 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) used in subways which is a form of an automatic train control system.

The system makes use of several distinct pulse train patterns of alternating current flowing through a track circuit to convey an aspect of the next signal. The circuit comprises the feedpoint at the next signal, one running rail, first locomotive axle, another running rail and back to the signal feedpoint. The resulting electromagnetic field is picked up by receiver coils located just front of the first axle of the locomotive. The signal is then amplified, filtered and evaluated. If the received signal changes from a more permissive to a less permissive aspect (like when passing a yellow light, indicating the next signal is red, which, in turn, requires stopping in front of it), an immediate vigilance control acknowledge is required; depending on configuration and aspects, periodic checks and a speed limit may be enforced as well.

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