Semiautomatic Block Signaling

Semiautomatic Block Signaling


a system of railroad automation and remote control that regulates train traffic and guarantees safety of operation. Here, some participation of personnel is required, which distinguishes this system from fully automatic block signaling.

In semiautomatic block signaling (Figure 1), the run between neighboring stations usually constitutes one block section, or protected section of the railway. Since only one train may occupy the block section at a given time, the dispatch of a train from one station to the next is possible only when the section is free. Permission to occupy the block section is given through a signal (a light or semaphore) controlled by the operator on duty at the station, who learns through the track circuit when a train has left a section.

Figure 1. Scheme of a relay semiautomatic block signaling system: (CP) control panels, (BD) blocking devices, (TC) track circuit, (S) sensors, (SLS) signal light sensors

A signal that the train has arrived at the station is sent to the next station by way of a blocking apparatus or control panels, which act upon the blocking devices and receive information from other devices monitoring the progress of the train along the controlled section. Both the arrival of a train within a section and its subsequent departure are recorded by sensors. This information is transmitted to the blocking devices and is then used to control the signals of the lights or semaphores along the tracks.

A semiautomatic block signaling system is considered electromechanical if the communication between stations is carried on by a mechanical blocking apparatus operated by electric current. If the communication is achieved by electrical relays, the system is called a relay system. Electromechanical semiautomatic block signaling systems utilize both light and semaphore signals; relay systems use light signals only.

The railroads of the USSR generally use relay systems for semiautomatic block signaling. In some sections, however, electromechanical systems have been preserved. In the relay systems, the operator transmits the commands and controls the signals from his control panel, and the execution of these commands is monitored by the color of the triggered signal lights. The operator dispatches a train by a “proceed” signal on the exit light. The arrival of a train at the station actuates a signal pedal or a track rail circuit. These act as sensors and flash a “stop” signal on the entrance light. The blocking apparatus in neighboring stations operate such that if the blocking apparatus in one station is in the “proceed” state indicating that the train can be dispatched, the apparatus in the next station will be in the “stop” state until its block section is vacated.


Putevaia blokirovka i avtoregulirovka, 2nd ed. Moscow, 1974.