B) Calling-On Signal
A major requirement for clearing the main signal is that the track upto the next signal must be clear and the overlap distance (120 m) beyond the next signal must also be clear. However sometimes a situation arises when the track upto the next signal is not clear or sometimes the overlap may not be clear. In such a situation the main signal cannot be cleared but to minimize detention to the train at the signal a subsidiary aspect is provided which is smaller in size and mounted below the main signal. When calling on is given a timing sequence of 120 seconds is initiated. After the lapse of 120 seconds, the calling-on aspect lights up. Meanwhile, the main signal remains lit at Danger. This is to remind the driver to proceed cautiously and be prepared to stop short of any obstruction on the track.