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Two amber lights showing together on one signal head.
Also known as: Preliminary caution aspect, Two yellows, Double amber, Four-aspect caution
A double yellow is a preliminary caution: it means the next signal shows a single yellow, and the one after that is red. It exists because modern trains cannot stop within one signal section, so the warning has to start two sections early.
Indicates a preliminary caution on a four-aspect railway signal: the next signal displays a single yellow and the second is at danger.
Contexts: Public information
Exists because braking distance at modern speeds exceeds one signal section, so a single caution aspect would not give enough warning.
Contexts: Public information
Belongs to the four-aspect system used in Great Britain and some other networks; three-aspect signalling has no equivalent and other countries use different schemes entirely.
Contexts: Public information
Two circular amber lamps illuminated simultaneously on a vertical signal head, with the red and green lamps dark.