On 22/01/2019 15:20, Jethro_uk wrote:

I think railway signalmen were onto that over a century previously, no ?
Railway signals using a colour *and* position to indicate status.


Didn't they have to change the meaning of mechanical signals? Signal
lever flag down initially meant that it was safe to proceed but the
signals had a tenancy to freeze in that position in winter weather.??

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