A calling-on signal is used to allow a loco or train to move into a block section or a track within station limits, which is or may be already be occupied by another train or loco. This is done for the purposes of coupling trains, for a train to enter a track for a long platform which already has another train stopped at it, for a train to enter station limits and wait behind another train on the section (thereby clearing the block section to the rear for another train to be received from the station in the rear), etc.
It always occurs in combination with a stop signal. It has only two positions, on and off. When on, the indication of...
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the stop signal applies. The calling-on signal can be off when the stop signal is at Stop; this shows the indication of Proceed Slow, which allows the train to pass the signal at low speed, after stopping, being prepared to stop for any vehicle or obstruction ahead of it on the same track.
In suburban sections a calling-on signal is sometimes used to allow EMU trains to proceed with caution on to a section of track occupied by another train. Often, there is electrical circuitry in the tracks to ensure that the calling-on signal does not change to off unless the train has come to a complete halt first. A calling-on signal may in some circumstances also be used to allow a train to pass a defective stop signal.
The calling-on signal is not pulled off when the stop signal is not on; and if a shunt signal is on the same pole below it, the shunt signal and calling-on signal cannot be pulled off at the same time.