Humble pie
After looking at the script for Sound Horn,<kuid2:60850:23113:4>, I discover I was in error to claim that an object in the trigger class - such as the Sound Horn sign - cannot be direction sensitive. The script requests the approaching train that has entered the Sound Horn's trigger area to perform a forward TrackSearch and determine, once the Sound Horn sign is found, whether it is approaching from the sign's 'Face' side or 'rear' side.
For mrmom's purpose, the script would need to be modified to control a light rather than sound a horn. A user interface to select the light that is to be controlled would also need to be developed.