blue traffic light

when I was a little boy, it always struck me that traffic lights should have a blue light in addition to the red and the green (and the amber), though I could never decide what it should be for.

anyway I discovered that in Hong Kong, the MTR East Rail line *does* have a blue light on its signals, so my childhood desire is finally satisfied. it even seems a sensible use:

Passenger trains have been running under an overlaid Automatic Train Protection (ATP) system since 1998. A blue aspect is displayed at signals when a train fitted with working ATP is approaching. A signal showing a blue aspect can be ignored by the driver, who will drive according to the information shown on the cab display.