Automatic Block Signaling

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Automatic Block Signaling


on railroads a system for regulating train traffic and for dividing railroad runs by light signals that change automatically through the action of moving trains. Under this system the runs between stations are divided by automatic communication light signals into block sections 1–3 km long; the distance between the light signals is the minimum distance for the safe simultaneous traffic of trains. The automatic action of the intermediate light signals within each block section is brought about by electric track circuits separated by insulating joints. One end of each track circuit is joined to the source of power and the other to a rail relay switch that controls the light signals through a contact system. If the block section is free, the current is fed from the battery by the rails through the winding of the track relay, thus turning the light signal to “proceed” (green). When the first wheels of a train make contact with the track circuit, the track relay turns the light signal to “stop” (red). The track circuits also monitor the conditions of the rails: if any rail is broken or missing, the track relay turns on and turns the light signal to red.

In the USSR, DC-wire automatic block signaling is used on lines for diesel locomotives and wireless AC coded automatic block signaling on lines for electric locomotives. Under the wire system the contact between the trackside light signals is established by two-wire line circuits which are joined to line relays. Under the code system the track-side light signals are connected by track circuits with signal codes: g for green, y for yellow, ry for red with yellow.

Automatic block signaling in and of itself does not exclude the possibility of a train’s passing a red light if conditions make it difficult to see the light or if the locomotive engineer is negligent. Therefore, automatic block signaling is supplemented with self-regulating devices, such as continuous automatic locomotive signaling and automatic stopping devices. Together, these devices make up a single automatic regulating system, which is installed on all major railroad lines in the USSR. This system increases the safety of train traffic and the capacity of the runs over the old systems (staff system, telegraph system, and semiautomatic block signaling) by 20–30 percent on single track lines and 2–3 times on double track lines. Automatic block signaling has been used on railroads since 1910 (USA) and in the USSR since 1930.


Kazakov, A. A. Avtoblokirovka, avtomaticheskaia lokomotivnaia signalizatsiia i avtostopy. Moscow, 1964.
Putevaia blokirovka i avtoregulirovka. Moscow, 1966.
Kazakov, A. A., and V. M. Davydovskii. Avtomatika, teleme-khanika i sviaz’ na zheleznodorozhnom transporte. Moscow, 1967.


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