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?

Railway signals in modern times have always had red, yellow (not
amber) and green. A four-aspect signal has two yellows - the sequence
approaching a stop signal goes G, YY, Y, R. There can be a number of
other indications as well as the main signal but these are almost
invariably white.
There's a number of uses for blue and purple, but not appearing with the
R/Y/G "traffic light" signals, at least in the UK. If you want to look
at the full range of signs and signals, http://www.railsigns.uk/ has a
very comprehensive guide.

Mike