On Thu, 13 Dec 2018 17:01:02 -0000, Marland wrote:

Fred Johnson wrote:
Can anybody else remember blue traffic lights on railways? Can't find
any evidence on google. I'm sure whereas cars have red/amber/green,
railways always had a 4th blue light. What does it mean and why has it
disappeared from Google?

Two possibilities but the first doesn’t really fit with your interpretation
of a 4 th aspect.
The glass in semaphore signal arms could be seen as blue if viewed in a
certain light as the the lens was a bluish glass to when illuminated at
night from an oil lamp or in later years a lowish watt electric lamp the
yellow flame /filament colour combined with the blue to indicate green..
The other is that you have seen or are referring to somewhere other than
the UK where blue is sometimes used.

This was definitely in the UK, and I remember it as a lit blue electric lamp, one of 4 lamps on the column. It was very blue and not green at all. This was about 30 years ago. I vaguely remember asking someone at the time and he agreed that there were blue lamps and they meant something different to the red/amber/green on roads, but he wasn't sure what. The only reference I can find to blue lights on a google search are seperate ones indicating the edge of a tunnel etc, but not part of a "traffic light" set.