TheThree Position Colour Light Signal now adopted as standard for New Zealand Railways has no mechanism. It consists merely of an iron box divided horizontally into three compartments, each compartment containing two (12 volt) lamps connected in parallel (to provide against lamp failure), and fronted by a coloured lens surmounted by a hood. the colours are green, yellow and red, in descending order. the hood is to provide a shadow by day, thus enhancing daylight visibility.

The signal indication is given by merely switching on the light to whichever colour is called for, the other two lenses remaining dark. This iron box forming the head of the signal is elevated on a pole, or attached to a bridge, and is fitted with a lower red light (called a “marker”) which remains lighted. Its position (in line, or staggered) shows the class of signal; automatic, controlled automatic or absolute automatic.

The lenses used are compound, and consist of a diffusing “roundel”—to spread the light evenly over the focussing “roundel.” This then parallels the rays, thus giving equal diffusion of visibility both near and far, by night and by day. For these lenses a special glass is used from which spectrum rays foreign to the desired colour are eliminated. (Thus ordinary flashed red glass may not eliminate all green and yellow rays when examined in the spectrometer, but this defect does not exist in the lenses used.)

The double lamps behind each of the lenses are connected to three separate circuits, the wires passing down inside the pole to the track relay, or to a relay governed by the track relay. This relay (or combination of relays) switches in the green, yellow, or red light, according to the condition of the track ahead of the signal, which again is modified by the position of the next signal in advance.