Continuous Automatic Warning System
Encyclopedia
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
.
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.
The carrier frequency
of 50 Hz is modulated
with pulsed square wave
codes 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 DART
areas, a different carrier frequency of 83.3 Hz is used to avoid interference from 50 Hz
power line frequency
. There is also a greater range of codes because they also control the automatic train protection
on the DART units.
Diesel drivers have to select the appropriate carrier frequency when moving to or from an electrified area. "Carrier 1" (50 Hz) is used in non-electrified areas and "Carrier 2" (83.3 Hz) is used in electrified areas.
having returned the signal ahead to red in an emergency.
s are used for train detection (i.e., "mini-CTC
" areas).
CAWS does not act in the event of a signal being passed at danger
if the Red aspect has been acknowledged.
Cab signalling
Cab signalling is a railway safety system that communicates track status information to the cab, crew compartment or driver's compartment of a locomotive, railcar or multiple unit, where the train driver or engine driver can see the information....
and train protection system used in Ireland to help train drivers
Railroad engineer
A railroad engineer, locomotive engineer, train operator, train driver or engine driver is a person who drives a train on a railroad...
observe and obey lineside signals.
CAWS has been in use on Iarnród Éireann
Iarnród Éireann
Iarnród Éireann is the national railway system operator of Ireland. Established on 2 February 1987, it is a subsidiary of Córas Iompair Éireann . It operates all internal intercity, commuter and freight railway services in the Republic of Ireland, and, jointly with Northern Ireland Railways, the...
diesel trains since 1984. All NIR
Northern Ireland Railways
NI Railways, also known as Northern Ireland Railways and for a brief period of time, Ulster Transport Railways , is the railway operator in Northern Ireland...
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
Dublin Suburban Rail
The Dublin Suburban Rail network, , is a railway network that serves the city of Dublin, Ireland, most of the Greater Dublin Area and outlying towns...
and Intercity routes to Cork
Cork (city)
Cork is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. Cork has a population of 119,418, while the addition of the suburban...
, Limerick
Limerick
Limerick is the third largest city in the Republic of Ireland, and the principal city of County Limerick and Ireland's Mid-West Region. It is the fifth most populous city in all of Ireland. When taking the extra-municipal suburbs into account, Limerick is the third largest conurbation in the...
, and Athlone and as far as the border on the Belfast line
Dublin-Belfast railway line
The Dublin-Belfast main line is a major railway route in Ireland that connects Dublin Connolly station in the Republic of Ireland and Belfast Central station in Northern Ireland.-History:The railway line was built by three separate companies...
.
This system is of American origin, being originally supplied by US&S
Union Switch & Signal
Union Switch and Signal was a supplier of railway signaling equipment, systems and services in Pittsburgh, Pennsylvania. As of January 1, 2009, US&S is known as Ansaldo STS USA.-History:...
, then a division of Westinghouse Air Brake Company
Westinghouse Air Brake Company
The railway air brake was invented by George Westinghouse of New York state in 1869. Soon after, he moved to Pittsburgh, Pennsylvania, where he established the Westinghouse Air Brake Company on September 28, 1869...
(WABCO). Current equipment is supplied by SASIB, a division of Alstom
Alstom
Alstom is a large multinational conglomerate which holds interests in the power generation and transport markets. According to the company website, in the years 2010-2011 Alstom had annual sales of over €20.9 billion, and employed more than 85,000 people in 70 countries. Alstom's headquarters are...
.
Principles of operation
CAWS repeats the aspects shown by the lineside colour light signalsRailway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
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.
Technical details
The system consists of coded track circuits and on-board equipment. The coded track circuits transmit information about the signal aspect to the on-board equipment via two pick-up coils mounted on the front of the train, one over each rail.The carrier frequency
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
of 50 Hz is modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
with pulsed square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
codes according to the signal aspect. No code is generated for a red aspect. The system is therefore fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
, since loss of code will result in a red indication on the ADU.
- Codes: 50, 120 and 180 cpm (cycles per minute).
- Translated by the CAWS as Yellow, Double Yellow and Green.
In electrified DART
Dublin Area Rapid Transit
The Dublin Area Rapid Transit is part of the suburban railway network in Ireland, running mainly along the coastline of Dublin Bay on the Trans-Dublin route, from Greystones in County Wicklow, through Dublin to Howth and Malahide in County Dublin.Trains are powered via a 1500V DC overhead catenary...
areas, a different carrier frequency of 83.3 Hz is used to avoid interference from 50 Hz
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...
power line frequency
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
. There is also a greater range of codes because they also control the automatic train protection
Automatic Train Protection
Automatic Train Protection in Great Britain refers to either of two implementations of a train protection system installed in some trains in order to help prevent collisions through a driver's failure to observe a signal or speed restriction...
on the DART units.
- Codes: 50, 75, 120, 180, 270 and 420 cpm.
- Translated by the CAWS as Yellow, Green, Yellow, Green, Double Yellow and Green.
- Translated by the DART ATP as 29 km/h, 30 km/h, 50 km/h, 50 km/h, 75 km/h and 100 km/h speed limits.
Diesel drivers have to select the appropriate carrier frequency when moving to or from an electrified area. "Carrier 1" (50 Hz) is used in non-electrified areas and "Carrier 2" (83.3 Hz) is used in electrified areas.
Advantages
Because CAWS is a continuous system, an upgrade 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. Similarly, the driver can quickly act upon an unexpected downgrade, which may be the result of the signalmanSignalman (rail)
A signalman or signaller is an employee of a railway transport network who operates the points and signals from a signal box in order to control the movement of trains.- History :...
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 counterAxle counter
An axle counter is a device on a railway that detects the passing of a train in lieu of the more common track circuit. A counting head is installed at each end of the section, and as each axle passes the head at the start of the section, a counter increments...
s are used for train detection (i.e., "mini-CTC
Centralized traffic control
Centralized traffic control is a form of railway signalling that originated in North America and centralizes train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that...
" areas).
CAWS does not act in the event of a signal being passed at danger
Signal passed at danger
A Signal passed at danger , in British railway terminology, occurs when a train passes a stop signal without authority to do so. It is a term primarily used within the British Railway Industry, although it can be applied worldwide.-Categories of SPAD:...
if the Red aspect has been acknowledged.
See also
- Automatic Train ProtectionAutomatic Train ProtectionAutomatic Train Protection in Great Britain refers to either of two implementations of a train protection system installed in some trains in order to help prevent collisions through a driver's failure to observe a signal or speed restriction...
- Automatic Warning SystemAutomatic Warning SystemThe Automatic Warning System is a form of limited cab signalling and train protection system introduced in 1956 in the United Kingdom to help train drivers observe and obey signals. It was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system...
- Pulse code cab signalingPulse code cab signalingPulse code cab signaling is a form of cab signaling developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad in the 1920s...