3. Subsidiary a signals

Subsidiary signals

Subsidiary signal below stop arm (lower quadrant) Subsidiary signal below main arm (upper quadrant)

Where it is necessary to give a driver special information about the section ahead, a subsidiary signal may be mounted below a stop arm. These consist of a small red arm with horizontal white stripe. When off, a letter C, S or W will be displayed, which indicate which of three functions apply:

  1. C - Calling-on signal. This indicates that the section ahead is occupied, and only applies on permissive block lines, station areas and other special locations.
  2. S - Shunt-ahead signal This indicates that the signal may be passed only for shunting purposes, only by the necessary distance to achieve such a shunt. Only provided on section signals
  3. W - Warning signal. This indicates Section Clear but Station or Junction Blocked, an old-fashioned term found in Regulation 5. Under normal block working conditions, an additional ¼ mile safety margin is allowed when a train is accepted into a block section, but at specially defined locations this requirement is waived. This signal instructs the driver to proceed through the block section with caution.

Colour light stop signal with subsidiary indication
Lower quadrant Upper quadrant Colour Light