Gives the driver advance warning that the stop signal ahead may be displaying 'danger'. The distant signal is installed roughly at full braking distance on approach to the stop signal to which it applies, taking into account the gradient, the permitted speed, and the braking performance of trains. On sighting a 'caution' aspect, the driver must prepare to stop at the signal ahead.

reference:
https://en.wikipedia.org/wiki/Application_of_railway_signals#Distant_signal [accessed 24Sep2019]

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