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Automatic Warning System (AWS) is a railway safety system invented and predominantly used in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution.
Depending on the upcoming signal state, the AWS will either produce a 'horn' sound (as a warning indication), or a 'bell' sound (as a clear indication). If the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS; if the driver correctly acknowledges the warning indication, by pressing an acknowledgement button, then a visual 'sunflower' is displayed to the driver, as a reminder of the warning.

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