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.

GH