Continuous Automatic Warning System

The Continuous Automatic Warning System (CAWS) is a form of cab signalling and train protection system used in Ireland to help train drivers observe and obey lineside signals.

CAWS has been in use on Iarnród Éireann diesel trains since 1984. All NIR trains operating cross-border must also be fitted with CAWS. Coded track circuits, which form the basis of the system (see below), are installed on all Dublin Suburban and Intercity routes to Cork, Limerick, and Athlone and as far as the border on the Belfast line.

This system is of American origin, being originally supplied by US&S, then a division of Westinghouse Air Brake Company (WABCO). Current equipment is supplied by SASIB, a division of Alstom.

CAWS repeats the aspects shown by the lineside colour light signals on an aspect display unit (ADU) inside the driver's cab. The ADU continuously displays the aspect that was shown by the previous signal until updated about 350 metres before the next signal. The ADU then displays the aspect shown by that signal.

A change of ADU display to a less restrictive aspect (e.g., double yellow to green) is termed an upgrade, while a change to a more restrictive aspect (e.g., single yellow to red) is called a downgrade. Any change of ADU display is accompanied by an audible indication. A momentary audible ‘warble’ sound indicates an upgrade. A downgrade is accompanied by a continuous audible tone and the illumination of the Acknowledge Switch that must be pressed by the driver within 7-seconds to prevent an automatic brake application occurring for one minute. This is not recoverable until the time has expired. Acknowledgement by the driver within the first 7-seconds immediately silences the tone.

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