A system of signalling, for double lines, which uses a device known as a 'Block Instrument', which is installed in the signal boxes at each end of the section. These instruments are interconnected and thus give a signalman an indication of the section of line they apply to.
The object of this system is to prevent more than one train being in the section, between the two signal boxes, at one time.
These instruments when used allow for transmission and reception of train working signals for each line. These train working signals are given by means of 'Bell' signals.
For example, the signal box at the start of the section would send the signal
3 pause 1, which is asking 'Is the line clear for:- an Express Passsenger,Fast Passenger.
The answer would be given by the receiving signal box sending
3 pause 1, this signifying 'the line is clear for Express Passenger, fast passenger.'
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