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:
- 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.
- 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
- 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.
