31. A railway signal shall be provided on each railway approach to the crossing and shall, when lit, show an intermittent white light. The white light shall only show if on each side of the railway at least one of the intermittent red lights in each of the traffic light signals mentioned in paragraph 20 above is lit, the main power supply has not failed and both barriers have commenced to fall. There shall be an additional railway signal on each railway approach mounted on the same post as the intermittent white light which shall, when lit, show an intermittent red light. This red light shall always be lit except when the red light mentioned above is showing.