Continuous Automatic Warning System
The Continuous Automatic Warning System (CAWS) is a form of cab signalling used in the Republic of Ireland to help train drivers observe and obey lineside signals.
CAWS has been in use on Irish Rail since the DART works in the mid-1980s. Its use is restricted to the busiest mainlines with modern colour light signalling. Originally supplied by WABCO of Pittsburgh, it uses the same technology as was earlier applied to some American railroads.
Principles of operation
CAWS is part of the signalling system and 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, even after it has been returned to a red aspect when the train passes it. When the train reaches a point (usually 350 metres) on the approach to 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 upshift, while a change to a more restrictive aspect (e.g. single yellow to red) is called a downshift. Any change of ADU display is accompanied by an audible indication. A warbling sound indicates an upshift. A downshift is accompanied by a warning horn that must be acknowledged by the driver within 7 seconds to prevent an automatic brake application. Acknowledgement by the driver immediately silences the horn.
Technical details
Information about the signal aspect is conveyed to the train via coded track circuits. The carrier frequency is modulated according to the signal aspect. No code is generated for a red aspect. The system is therefore fail-safe, since loss of code will result in a red indication on the ADU.
In electrified areas, a different carrier frequency is used. Also, there is a greater range of codes because they also control the automatic train protection on the DART units. Drivers have to select the appropriate carrier frequency when moving to or from an electrified area ("C1" for a non-electrified area, "C2" for an electrified area).
Advantages
Because CAWS is a continuous system, an upshift can occur anywhere between signals. The driver may then accelerate in the knowledge that the signal ahead has changed to a better aspect, even though it may not yet be visible to him. Similarly, the driver can quickly act upon an unexpected downshift, which may be the result of the signalman having returned the signal ahead to red in an emergency.
Disadvantages
Since CAWS is dependent on track circuits for its operation, it cannot be applied where axle counters are used for train detection (i.e. "mini-CTC" areas).
CAWS does not act in the event of a signal being passed at danger.
See also
 | This rail-transport related article is a stub. You can help Wikipedia by expanding it. |