The Continuous Automatic Warning System (CAWS) is a form of

cab signalling Cab signaling is a railway safety system that communicates track status and condition information to the cab, crew compartment or driver's compartment of a locomotive, railcar or multiple unit. The information is continually updated giving an ...

and

train protection system A train protection system is a railway technical installation to ensure safe operation in the event of human error. Development Train stops The earliest systems were train stops, as still used by the New York City Subway, the Toronto subway, t ...

used in

Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...

to help

train drivers A train driver, engine driver, engineman or locomotive driver, commonly known as an engineer or railroad engineer in the United States and Canada, and also as a locomotive handler, locomotive operator, train operator, or motorman, is a pers ...

observe and obey lineside signals. CAWS has been in use on

Iarnród Éireann Iarnród Éireann () or Irish Rail, is the operator of the national railway network of Ireland. Established on 2 February 1987, it is a subsidiary of Córas Iompair Éireann (CIÉ). It operates all internal InterCity, Commuter, DART and fr ...

diesel trains since 1984. All NIR trains operating cross-border must also be fitted with CAWS. Coded

track circuits A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters. Principles and operation The basic principle behind t ...

, which form the basis of the system (see below), are installed on all Dublin Suburban and Intercity routes to

Cork Cork or CORK may refer to: Materials * Cork (material), an impermeable buoyant plant product ** Cork (plug), a cylindrical or conical object used to seal a container ***Wine cork Places Ireland * Cork (city) ** Metropolitan Cork, also known as G ...

Limerick Limerick ( ; ga, Luimneach ) is a western city in Ireland situated within County Limerick. It is in the province of Munster and is located in the Mid-West which comprises part of the Southern Region. With a population of 94,192 at the 2016 ...

, and

Athlone Athlone (; ) is a town on the border of County Roscommon and County Westmeath, Ireland. It is located on the River Shannon near the southern shore of Lough Ree. It is the second most populous town in the Midlands Region with a population of ...

and as far as the border on the Belfast line. This system is of American origin, being originally supplied by

US&S Union Switch & Signal (commonly referred to as US&S) was an American company based in Pittsburgh, Pennsylvania, which focused on railway signaling equipment, systems and services. The company was acquired by Ansaldo STS (from 2015, Hitachi Rail ...

, then a division of

Westinghouse Air Brake Company The Westinghouse Air Brake Company (sometimes nicknamed or abbreviated WABCO although this was also confusingly used for spinoffs) was founded on September 28, 1869 by George Westinghouse in Pittsburgh, Pennsylvania. Earlier in the year he had i ...

(WABCO). Current equipment is supplied by SASIB, a division of

Alstom Alstom SA is a French multinational rolling stock manufacturer operating worldwide in rail transport markets, active in the fields of passenger transportation, signalling, and locomotives, with products including the AGV, TGV, Eurostar, Avelia ...

Principles of operation

CAWS repeats the aspects shown by the lineside colour light

signals In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

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 seven 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 seven seconds immediately silences the tone.

Technical details

The system consists of coded

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 50 Hz

carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...

modulated In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...

with pulsed

square wave A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...

codes according to the signal aspect. No code is generated for a red aspect. The system is therefore

fail-safe In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...

, 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 Dart or DART may refer to: * Dart, the equipment in the game of darts Arts, entertainment and media * Dart (comics), an Image Comics superhero * Dart, a character from ''G.I. Joe'' * Dart, a ''Thomas & Friends'' railway engine character * Dar ...

areas, a different

carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a m ...

of 83.3 Hz is used to avoid interference from

50 Hz The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to th ...

power line frequency. There is also a greater range of codes because they also control the

automatic train protection Automatic train protection (ATP) is a type of train protection system which continually checks that the speed of a train is compatible with the permitted speed allowed by signalling, including automatic stop at certain signal aspects. If it is ...

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

signalman A signalman is a person who historically made signals using flags and light. In modern times, the role of signalmen has evolved and now usually uses electronic communication equipment. Signalmen usually work in rail transport networks, armed for ...

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 counter An axle counter is a system used in railway signalling to detect the clear or occupied status of a section of track between two points. The system generally consists of a wheel sensor (one for each end of the section) and an evaluation unit for c ...

s are used for train detection (i.e., "mini- CTC" areas) without the additional cost. CAWS does not act in the event of a signal being passed at danger if the Red aspect has been acknowledged.

References

* EU Directive 96/48/EC, Document 96/48-ST12 part 3, ''Interoperability of the trans-European high speed rail system'' {{Railwaysignalling Train protection systems Rail infrastructure in Ireland

Principles of operation

Technical details

Advantages

Disadvantages

See also

References