A System of Broken Rail Detection

The Institution of Railway Signal Engineers Inc Australasian Section Incorporated

A SYSTEM FOR BROKEN RAIL DETECTION INDEPENDENT OF THE SIGNALLING SYSTEM Rebecca Taylor B. Eng (Hons) Mech Signals Engineer, Public Transport Authority of Western Australia

SUMMARY This paper considers the problem of detecting breaks in a rail. It provides a review of types of rail break, which types need to be detected and why their detection is necessary. It also tackles the question of where the responsibility for detecting broken rails lies. Maintenance and management of the rail and track assets are the responsibility of the track maintenance group. Hence detection of conditions relating to the rail must therefore fall within that scope. Further to this, Signalling systems cannot be relied upon to detect all types of rail break. Signalling systems employing communications based train position detection or axle counters have no mechanisms whatsoever for detection of broken rails. This paper proposes a possible system that may be able to provide a better solution for detection of broken rails than traditional signalling systems and can do it independently of the signalling system.

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detection may be desired in additional to the normal track maintenance processes.

INTRODUCTION

The problem of detecting broken rails needs to be considered carefully. It is possible that a broken rail may lead to a derailment with potentially catastrophic results. The intended purpose of the system and the characteristics of the broken rail that it is designed to detect should be defined. While the responsibility for the condition of the rail lies with the track maintenance group, existing signalling systems have been seen as able to provide some level of detection of rail breaks. When considered more thoroughly, it is clear that signalling systems have many limitations in this function. Signalling systems that do not use track based train position detection have no mechanisms for detection of broken rails. As such the signalling system cannot be relied upon to detect broken rail.

A separate system is proposed in this paper that may be capable of overcoming the shortfalls of using track circuits for broken rail detection. 2 2.1

Ultrasonic testing provides early warning of anomalies within the rail. This provides mitigation against the risk of derailment due to a broken rail. However, it also provides additional information that assists with management of the asset life of the rail. Early treatment of rail defects can also reduce rail and wheel wear resulting from the impact of poor rail condition. For further risk mitigation of a derailment due to broken rail, a continuous system of broken rail

IRSE Australasia Technical Meeting: Sydney

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DETECTION OF BROKEN RAIL Defining the problem When tackling the problem of detecting breaks in the rail, consideration needs to be given to what type of break can occur, which types should be detected and why detection is necessary. There are a number of ways in which the rail may break. A list of common types of rail break is shown in figure 1 shown on page 2 below. Further to this, some common rail defects include squats, wheel burn, inclusions and surface cracking. Rails are subjected to very large forces even when operated under normal traffic conditions and exposed to potential extremes of temperature from our environment. Under these conditions, all of the minor defects listed can propagate into the types of breaks shown in figure 1 if left untreated. The next consideration is what consequence is to be avoided such that 18 March 2011

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A SYSTEM FOR BROKEN RAIL DETECTION INDEPENDENT OF THE SIGNALLING SYSTEM

detection of these rail breaks is necessary. A major reason for detecting breaks is to mitigate the risk of a train derailment resulting from a broken rail. However, other reasons for detecting rail breaks include improving train performance in terms of the quality of the ride, and extending the asset life of both the rail and the wheels of any rollingstock running on it.

traditional track circuits afford features that are able to detect some types of broken rails. However, there are a number of limitations. Four of the six types of rail breaks shown in figure 1. do not break the electrical continuity of the track circuit and hence would remain undetected. Earthing and bonding procedures, particularly in electrified territory, may require very frequent bonding of the rail infrastructure to the earth system. This has the potential to provide an alternative path for the track circuit current, thus by-passing rail breaks that may otherwise have been detected. Hence track circuits cannot be relied upon to detect rail breaks in electrified rail territory. Traditional track circuits also have limitations on length before they become unreliable for operational purposes. This makes them relatively infrastructure intensive as a system, especially as their installation requires equipment at both ends of each track circuit. This means that power must be distributed to a number of sites necessitating significant trenching. Another limitation is that the track circuit can only indicate that the track is not clear for the passage of a train. It cannot differentiate between rail breakages and other reasons such as bonding failures resulting in the track showing occupied. As such, human intervention is required to attend the site and determine the reason for the track circuit showing occupied. This can be time consuming and requires that trains will be delayed for safety reasons until the cause of the track “occupancy” is determined.

Figure 1. Types of rail breaks 2.2

In some railways alternate signalling systems, or possibly manual systems such as train orders, may be employed to avoid the need for the large amounts of infrastructure required with traditional track circuited systems. These alternative signalling technologies such as axle counters or communications based systems have no mechanisms for detection of rail breakage at all as they make use of non track-based methods of train position detection.

Whose Problem is it? Maintenance and management of the rail and track formation is the sole responsibility of the track maintenance group. By contrast, the signalling system is responsible primarily for the safe separation of trains. It is therefore clear that responsibility for detecting rail breaks must lie with the track maintenance group and not the signalling system.

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Signalling Systems’ Broken Rail Detection

Capability

for

While the responsibility definitely lies with the track maintenance group, track based train position detection systems such as IRSE Australasia Technical Meeting: Sydney

Signalling systems cannot be relied upon to detect all rail breaks and some systems cannot detect rail breaks of any sort. 2.4

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Track Maintenance Considerations To achieve the aims of better ride performance and extended asset life of the 18 March 2011

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rail and rollingstock, detection and treatment of the minor defects before they progress into more significant rail breaks is more effective. Treatment of defects at this stage can prevent the wear and damage that leads to an eventual rail break. Thus the need to replace whole sections of rail can be significantly deferred. Considerable wear on rollingstock wheels and bogies can also be avoided, which in turn reduces rail wear that would have resulted from the impact of the worn rollingstock on the rail.

into one rail with a signature, and receiving it back from the other rail with a different signature could allow broken rail detection using many ‘track circuits’ each several kilometres in length without the use of insulated joints. Rail integrity is then proved by receiving the correct signature. This system would need further development to resolve the issue of alternate paths for the injected current caused by earthing and bonding, particularly if it were to be used in electrified areas.

In order to detect these minor defects ultrasonic testing can be conducted on a regular basis. This gives the track maintenance group a wealth of information about the condition of the rail. Treatment of defects can then be planned accurately and efficiently thus minimising disruption to rail services, extending the life of the rail, and minimising the likelihood of a rail break occurring.

With a fault causing a short-circuit present it will allow the position of the fault to be determined using comparison of phases of the current and voltage of the injection signal, using the same philosophy as predictor technologies. This means that fault response crews can receive notification including accurate details of the position of the fault, allowing faster maintenance response.

A proposed Broken Rail detection Device

The system would need to be attached to a central SCADA interface enabling alarms to be sent to the appropriate personnel. Alarms could be sent via email and SMS. SMS alarms that escalate along an established hierarchy until acknowledged would negate the requirement for continuous monitoring of the alarm system. It also avoids the problem of yet more alarms requiring to be monitored by the train controllers.

Use of ultrasonic testing can be expensive and it may not be cost effective. For some railways a further safe guard may be sought against derailment due to broken rail. Under these circumstances a track based detection system may provide some added opportunity to respond to breaks or defects prior to any accident occurring. A possible solution for a broken rail detection system may be developed which makes use of track circuits which have the transmitter and receiver at the same location (such as Westrak or GCP technology). This means that the active components can all be installed at the same site for each circuit reducing the numbers of sites which need distributed power and associated trenching. This would considerably reduce the installation costs. This style of track circuit used as a broken rail detector has a failure mode in which a break in the rail is masked if there is any part of the track bypassed by an alternate path for the track circuit current. This may occur under fault conditions depending on the earthing and bonding philosophy employed. Additional features would need to be added to such an arrangement to detect a break anywhere within the circuit. This could be overcome by the use of an injected current modulated by one frequency into the rails, and at the far end of the track circuit place a narrowband shunt with a device that alters the modulation on the return rail. An arrangement involving injection of current IRSE Australasia Technical Meeting: Sydney

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Detected defects could be categorised by predetermined parameters set on calibration of the circuits during set-up. The impact on train running of each level of defect could then be assessed and the appropriate action taken. Lower level alarms could denote defects that serve as early warning but would not yet need more cautious train running. These alarms could be sent directly to the track maintenance crew to allow inspection and planning of remedial treatment. Higher level alarms could be for more serious defects that need immediate inspection and perhaps a reduction in train speed. This speed reduction could be applied by the train controllers on receipt of the alarm as a predetermined action until such time as the track maintenance crew is able to make an assessment of the rail condition. This type of response would mean that train services can still run and delays would not be as large as a signal placed at red and the requirement to authorise trains to pass the signal. Complete rail separation would be treated as the highest level of alarm and should be sent to the train controller and an input into 18 March 2011

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the interlocking placing the signal reading over the break at red. The train controller would be able to determine that the signal is at red due to the notification of the rail break and so would be able to call on the track maintenance crew for emergency fault response rather than calling the signals maintenance crew who would then call the track crew potentially after some wasted fault finding time. It may be practical for a train to be authorised to pass the signal at stop and proceed with due caution. This would allow further information to be gathered by the driver and relayed to the maintenance staff as to whether there really was a rail break or the equipment had failed. Track maintenance crews are generally not staffed to maintain electronic equipment so it may be convenient for routine maintenance and fault response to be undertaken by the signalling maintenance crew. In order to assist with determining the appropriate group to respond to the alarm, a health status of the system could be included in the device. If the health of the system is OK, then a high level alarm should be attended by the track crew. If the health of the system is not OK then the signals crew should attend. The signals crew should also make a quick inspection of the track under these circumstances to ensure no major rail breaks are present. The alarm history should be available to both groups to highlight any trends and assist with root cause analysis of any faults.

not apply. This could allow more flexibility for detection circuits to be made more sensitive. Breaks will affect the characteristics of the rail, particularly if there is a substantial reduction in the cross sectional area of the material that remains electrically continuous. However, further research would need to be conducted to determine whether the material characteristics of fractured rail are detectable in a practical sense when the device is in service in the field. Once detected the location of the anomalies could be pin pointed by making a comparison of the phase angles of the voltage and current of the transmitted signal. The system should also be able to determine differing types of anomalies by making a comparison of the received signal based on predetermined signal forms and the level to which the received signal differed from the normal signal. 2.6

The system will not compromise the safety of the signalling system as any inputs from the broken rail detection system should be such that a lack of OK input should cause signals to show a red aspect. However, to reduce the incidence of equipment failure leading to lack of availability of the signalling system, the broken rail detection system should be designed to have as high a mean time between failure as possible. The ability of the broken rail detection system to identify minor flaws and fractures should give time to repair the minor defects before they escalate into a more major defects that might impact on the signalling system or train services. The broken rail detection system would not be providing information to the signalling system regarding train position, but instead providing early warning to the track maintenance crew. The extremes of absolute fail safety in terms of train position detection on one hand and operational availability on the other which normally dictate the track set-up parameters would IRSE Australasia Technical Meeting: Sydney

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Current Developments A brief review of current developments in the area of broken rail detection has identified a number of registered US patents. However, these are largely for devices that provide broken rail detection as a by-product of train position detection. The Transportation Research Board, in conjunction with the American Association of Railroads and the Transportation Technology Centre Inc. (TTCI), are currently considering the problem of broken rail detection. The requirement for implementation of Positive Train Control on American railways particularly creates the problem of broken rail detection no longer being achievable at all with the signalling system equipment. The vast distances covered by American railways also makes infrastructure intensive systems uneconomical. TTCI ran a workshop to identify possible technical solutions for evaluation. They have also specifically evaluated three systems and undertaken field trials on two of those systems. The first system utilised a single optic fibre glued along the length of the rail. A signal was then transmitted through the fibre. Any rail breaks cause the fibre to break and hence the transmitted signal is disrupted. This system gives immediate detection of a break. Unfortunately, a fibre break means the replacement of a complete run of fibre to the nearest splice points.

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The second system consisted of a network of selfcontained strain gauge units that transmit strain readings back to a central base station. This system can give warning of rail buckling as well as rail breaks. However, it has a discrete sample time rather than being continuous. It is also susceptible to damage during rail heating operations if care is not taken by maintenance staff.

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I would like to acknowledge the Public Transport Authority of Western Australia for permission and support to prepare this paper and contribute to this meeting. In particular, Terry Macdougall has generously contributed the original ideas and a sounding board for this paper. 5

The third system evaluated was not sufficiently developed at the time to allow field trials to be conducted. A theoretical evaluation of the system was conducted. The system compared the level of traction return current in each of the running rails. A broken rail would cause an imbalance in the currents running in each of the rails which would be detected by a device making such a comparison. This system relies on the presence of traction return current and so is well suited to electrified railways. However, longer distance railways are usually not electrified due to the amount of infrastructure that would be required. Hence this system would not be suitable for large regional railways. 3

ACKNOWLEDGEMENTS

CONCLUSION

REFERENCES 1. Reiff,R. and Davis,D. (2000), TTCI testing on-track circuit-based broken-rail detectors - Transportation Technology Centre Inc, Railway Track and Structures, April 2000, viewed March 6 2011, http://findarticles.com/p/articles/mi_m0BF W/is_4_96/ai_62448990/?tag=content;col 1 2. Transportation Research Board (2001), Transit Cooperative Research Program Report 71 Track-related Research Vol1.,p1-39, National Academy Press, Washington DC 3. Voran Consultants (1998), South West Metropolitan Railways Broken Rail Detection, Investigations and Study, p29, Report for Department of Transport

Management and maintenance of the rail is the responsibility of the track maintenance group. Signalling systems that employ track based train position detection can provide very limited detection of rail breaks. However, they cannot detect any breaks in which the electrical continuity of the circuit is maintained. For this reason signalling systems cannot be relied upon to provide effective mitigation of the risk of derailment due to a broken rail. Ultrasonic testing provides the added benefit of accurate data regarding minor defects. This data can enable more effective and efficient management of the rail asset life and with early treatment wheel and rail wear due to poor rail condition can be reduced. Ultrasonic testing may not be cost effective or a continuous method of detecting rail breaks may be desired to further mitigate the risk of a derailment. In these scenarios a system of broken rail detection circuits may be developed based on a circuit with the transmitter and receiver collocated and narrowband shunts installed across the ends of the circuit. If an injected signature is used with a modulating device at the ends of the circuit then analysis of the received signature can prove rail integrity and in the case of a break analysis of the signature received back could provide information regarding the nature and location of the anomaly.


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AUTHOR

Rebecca has worked for the Public Transport Authority of Western Australia for eight years. She initially joined the graduate program and then spent two years working in the signals and control systems areas as a graduate engineer. At the end of the two year program she gained a position as a Signals Engineer. In the course of her work at PTA Rebecca has been involved with maintenance of the current signalling system, review of system performance, and minor design and implementation of system improvements. She is a member of the SPAD committee and also represents the Signalling Engineering Manager at the intrastate operational level crossing subcommittee.

Rebecca Taylor


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A System of Broken Rail Detection

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