4 light signal red/green/2yellow standard square head This 4 aspect signal is placed at the side of the track. It is suitable for any layout based from around 1970 when this style of signal head came into use, and examples could still be found on the network until 2009, when the new slimmer signals became standard although this replacement programme began around 2000. A red light indicates stop, a double yellow light indicates caution the next signal is yellow and at caution, a single yellow light indicates caution the next light is red and Green indicates permission to proceed or route clear. This signal is ready to use out of the box and features a pre-wired resistor to allow the safe operation of the signal from a 12 Volt DC power supply.