Axle Counters vs Tracks Circuits

In the ever-evolving landscape of railway technology, two critical components stand at the forefront of train detection systems: axle counters and track circuits. These technologies play a crucial role in ensuring the safety and efficiency of rail networks worldwide. As railway operators seek to enhance their infrastructure, the choice between axle counters and track circuits has become a topic of significant interest and debate within the industry.

This article delves into the intricacies of axle counters and track circuits, exploring their basic principles, safety features, reliability, and cost considerations. By examining these key aspects, we aim to provide a comprehensive comparison of these two vital train detection methods. This analysis will shed light on the strengths and limitations of each system, helping railway professionals make informed decisions when upgrading or implementing new signalling solutions.

Basic Principles of Track Circuits

Track circuits are fundamental to modern railway signalling systems, playing a crucial role in ensuring safe and efficient train operations. These systems detect the presence of trains on specific sections of track, providing vital information to control train movements and prevent collisions.

How Track Circuits Work

The basic principle behind track circuits lies in the electrical connexion between the two rails created by the wheels and train axle of locomotives, rolling stock and rail vehicles. This connexion forms a short circuit, which is monitored by electrical equipment to detect the presence or absence of trains.

A typical track circuit consists of a power source connected to one end of a section of track and a relay connected to the other end . When no train is present, the current flows through the rails to energise the relay. However, when a train enters the section, its wheels and axles create a low-resistance path between the rails, shunting the current away from the relay and causing it to de-energise .

This change in the relay’s state is interpreted by the signalling system as an indication of track occupancy. The system then uses this information to control signals and prevent other trains from entering the occupied section .

Types of Track Circuits

There are several types of track circuits used in railway systems, each designed to suit specific operational requirements and environmental conditions.

1. DC Track Circuits: These are among the most common types, particularly in non-electrified sections . They use direct current and are simple in design but can be susceptible to interference from stray currents .
2. AC Track Circuits: These circuits use alternating current, typically at a frequency of 83.5 Hz to avoid interference from 50 Hz traction current . They are more resistant to stray currents and can be used on electrified tracks .
3. Audio Frequency Track Circuits (AFTC): These high-frequency circuits eliminate the need for insulated joints in the rails, reducing installation and maintenance costs . They operate at frequencies in the audio range, from 91 Hz up to 10 kHz .
4. Coded Track Circuits: These circuits modulate the track circuit current to transmit additional information, such as speed commands, to trains .

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Advantages of a Track Circuit

A Track circuit offer several advantages that have made them a cornerstone of railway signalling systems:

1. Fail-Safe Operation: Track circuits are designed to indicate the presence of a train when failures occur, ensuring safety even in the event of system malfunctions .
2. Continuous Detection: Unlike point detection systems, track circuits provide continuous monitoring of train presence along the entire track section .
3. Broken Rail Detection: By connecting the power source and relay to opposite ends of the section, track circuits can detect broken rails, enhancing safety .
4. Automatic Signalling: A Track circuit enables semi-automatic operation of signalling systems, reducing the reliance on human operators and improving overall system reliability .
5. Integration with Other Systems: Track circuits can be used to transmit additional information to trains, such as speed commands, enhancing the capabilities of train control systems .

Despite these advantages, it’s important to note that track circuits require careful design and maintenance to ensure reliable operation. Factors such as rail contamination, electrical interference, and environmental conditions can affect their performance. As railway technology continues to evolve, track circuits remain a critical component in ensuring the safety and efficiency of train operations worldwide.

Basic Principles of an Axle Counter

Axle counters have emerged as a robust alternative to a track circuit for train detection, offering significant advantages in various applications and conditions. These systems have been widely used in Europe and other parts of the world for decades, providing reliable train detection where track circuits may become unreliable, impractical, or too expensive to maintain .

How an Axle Counter System Works

Axle counters operate on a simple yet effective principle. The system consists of two main components: sensors and evaluators. The sensors, installed at each end of a defined track section, detect individual train axles using mechanical, electrical, or fibre optic methods. The evaluators, which are centrally located computers, process the information from the sensors to determine track occupancy .

When a train enters a section, the sensor at the entry point detects and counts each axle passing over it. As the train exits the section, the sensor at the exit point performs the same count. The evaluator then compares these counts. If the number of axles entering matches the number exiting, the section is considered clear .

Modern axle counters typically use electromagnetic principles for detection. Each sensor comprises two independent coils – a transmitter and a receiver. When a train wheel passes over the sensor, it alters the magnetic field between these coils due to the ferromagnetic properties of the wheel. This change is detected and registered as an axle count .

Types of Axle Counters

Axle counters have evolved over time, with different types developed to suit various operational needs:

1. Mechanical Axle Counters: Early versions used cradle-like mechanisms where a physical arm was actuated by the wheel flange .
2. Electromagnetic Axle Counters: Modern systems use contactless electromagnetic detection, reducing wear and tear and improving reliability .
3. Fibre Optic Axle Counters: These use light signals for detection, offering high precision and resistance to electromagnetic interference.
4. Digital Axle Counters: Advanced systems that provide additional features such as remote diagnostics and integration with digital railway systems .

Advantages of Axle Counters

Axle counters offer several significant advantages over traditional track circuits:

1. Ease of Installation and Maintenance: Axle counters are quicker and easier to install compared to track circuits. They require less cabling and eliminate the need for insulated rail joints, reducing installation complexity and maintenance requirements .
2. Cost-Effectiveness: With lower maintenance needs and longer track section capabilities, axle counters often provide lower life cycle costs .
3. Reliability in Challenging Environments: Axle counters perform well in extreme weather conditions and challenging environments such as wet tunnels, steel bridges, or areas with poor rail-to-ballast resistance .
4. Immunity to Rail Contamination: Unlike a track circuit, axle counters are not affected by railhead contamination due to rust, grease, or leaf residue .
5. Longer Section Lengths: Axle counters can cover longer track sections, up to 10,000 metres when connected directly, making them ideal for dark territory applications .
6. Versatility: They can be used in various applications, including grade crossings, switch point protection, and yard automation .
7. Overlay Capability: Axle counters can be overlaid on existing detection systems, allowing for easier upgrades and transitions .
8. Low Power Requirements: Axle counters generally have lower power needs compared to track circuits .

While axle counters offer numerous benefits, it’s important to note that they also have some limitations. For instance, when axle counter systems fail, they require a manual reset process to prove the section clear, which can be time-consuming . Additionally, there’s a potential for ‘wheel rock’ issues, where a wheel stopping directly over a sensor can cause a right-side failure .

Despite these challenges, the advantages of axle counters have led to their increasing adoption in modern railway signalling systems. Many countries now prefer axle counters for new schemes, with systems supplied by manufacturers such as Thales and Frauscher . As railway technology continues to evolve, axle counters are likely to play an increasingly important role in ensuring safe and efficient train operations.

Safety Comparison

When comparing axle counters and track circuits, safety is a paramount consideration. Both systems play a crucial role in ensuring the safe operation of railways, but they have distinct characteristics that impact their safety performance.

Fail-Safe Design

One of the fundamental safety requirements of a train control system is to prove that a section of track is clear of other traffic before authorising a train to proceed . Both axle counters and track circuits are designed with this principle in mind, but they achieve it through different means.

Track circuits operate on a fail-safe principle. They continuously prove the absence of a train from a given section of track . Any electrical short-circuit between the rails, whether caused by a train or not, or any disconnection within the circuit, will cause the track circuit to ‘fail’ and inform the signalling system that the track is occupied . This fail-safe design ensures that any fault in the system results in a safer state, preventing potential accidents.

Axle counters, while also designed for safety, have a different approach to fail-safe operation. When an axle counter system fails, for example due to a power supply problem, it loses track of the number of axles that have passed through it. In such cases, for safety reasons, it always recovers showing the section of line as occupied . This requires the section to be proved clear of trains before the axle counters can be restored and reset, which can be time-consuming but ensures safety .

Maintenance Safety

The maintenance requirements and safety implications of these systems differ significantly. Track circuits require frequent and regular maintenance to ensure reliable operation . This is because they are sensitive to environmental conditions such as heat, snow, water, road salt, rust, and debris . These factors can impact the system’s performance and potentially lead to safety issues if not properly addressed.

On the other hand, axle counters have lower maintenance requirements. They offer remote calibration and diagnostics capabilities, reducing the need for regular on-site maintenance . This not only reduces maintenance costs but also minimises the safety risks associated with frequent trackside work.

Furthermore, axle counters are virtually immune to many environmental conditions that affect track circuits . This resilience contributes to their overall reliability and safety performance, particularly in challenging environments where track circuits might struggle to operate consistently.

Detection of Broken Rails

One notable difference between the two systems lies in their ability to detect broken rails. Track circuits, while not specifically designed for this purpose, can detect many (but not all) kinds of broken rails . This capability is limited in AC traction areas and not possible in the common rail in DC traction areas .

By contrast, axle counters cannot detect broken rails at all . This limitation is significant, as broken rails pose a serious safety risk. However, it’s worth noting that improved rail integrity and ultrasonic testing have provided better methods for detecting rail problems, and the number of broken rails across networks has reduced dramatically .

In conclusion, both axle counters and track circuits have their strengths and limitations when it comes to safety. Track circuits offer fail-safe operation and some ability to detect broken rails, but require more maintenance and are susceptible to environmental factors. An axle counter, while unable to detect broken rails, offer high reliability, lower maintenance requirements, and better performance in challenging environments. The choice between these systems often depends on specific operational requirements and environmental conditions of the railway network.

Reliability and Availability

The reliability and availability of train detection systems are crucial factors in ensuring safe and efficient railway operations. Both axle counters and track circuits have distinct characteristics that impact their performance in these areas.

Mean Time Between Failures

Reliable, fail-safe train detection serves as the foundation for railroad signalling . When comparing axle counters and track circuits, the mean time between failures is a critical metric to consider. Axle counters have gained popularity due to their high uptime and overall reliability . This reliability stems from their simpler design and fewer components susceptible to environmental factors.

Track circuits, on the other hand, face several challenges that can affect their reliability. They are sensitive to deteriorated ballast conditions and rusty rails, which can lead to false readings or system failures . Additionally, track circuits can be impacted by power fluctuations, further compromising their reliability . These factors contribute to a potentially lower mean time between failures for track circuits compared to axle counters.

Ease of Maintenance

Maintenance requirements play a significant role in the overall availability of train detection systems. Axle counters have a clear advantage in this area, with low maintenance needs and the ability to perform remote calibration and diagnostics . This reduces the necessity for regular on-site maintenance, leading to lower life cycle costs and increased system availability.

In contrast, track circuits require frequent and regular maintenance schedules . This is due to their sensitivity to various environmental factors and the need to maintain insulated rail joints, which can be expensive and time-consuming . The high level of electrical power and advanced power supply equipment used in track circuits also necessitates frequent maintenance .

Performance in Adverse Conditions

The ability of train detection systems to perform reliably in challenging environments is crucial for maintaining high availability. Axle counters have demonstrated superior performance in extreme weather and environmental conditions . They can be used effectively in areas not conducive to track circuits, such as steel structures (bridges, elevated tracks) and embedded rail .

Track circuits, however, face reliability issues due to many unavoidable factors such as road salting, flooding, snow, debris, rusty rails, and other track and environmental conditions . These factors can lead to decreased reliability and potentially compromise safety if not addressed promptly.

To address these challenges, improvements in train detection systems are needed. These include:

1. Reliable performance in all environmental and deteriorated track/ballast conditions
2. Low power requirements
3. Infrequent maintenance needs
4. Quick and simple installation without drilling of rail
5. Capability for long block lengths
6. High uptime and improved safety
7. Seamless integration with existing infrastructure

Axle counters have made significant strides in meeting these requirements. They offer quick and easy installation, low maintenance (both in time and cost), low life cycle costs, and overall reliability in extreme conditions . Furthermore, axle counters can be used as an overlay for track vacancy detection in mass transit systems and CBTC Fallback .

In conclusion, while both systems have their merits, an axle counter generally offer higher reliability and availability compared to track circuits. Their simpler design, lower maintenance requirements, and better performance in adverse conditions contribute to increased uptime and overall system efficiency. However, it’s important to note that track circuits still hold an advantage in one crucial area: the instant detection of a break in a section of track . This capability remains a significant factor in the ongoing use of track circuits in many railway systems.

Cost Considerations

When evaluating train detection systems, cost considerations play a crucial role in decision-making for railway operators. Both axle counters and track circuits have distinct cost profiles that impact their overall value proposition.

Initial Installation Costs

The initial installation costs for axle counters and track circuits differ significantly. Axle counters have gained popularity due to their quick and easy installation process . This advantage translates to lower upfront costs, as the installation requires less time, labour, and materials compared to track circuits.

In contrast, track circuits have higher initial installation costs . This is primarily due to the more complex infrastructure required, including the need for insulated rail joints and extensive cabling. The installation process for track circuits is generally more time-consuming and labour-intensive, contributing to higher upfront expenses.

Ongoing Maintenance Costs

Maintenance costs are a critical factor in the long-term financial viability of train detection systems. Axle counters have a clear advantage in this area, with significantly lower maintenance requirements both in terms of time and cost . This reduced need for maintenance stems from their simpler design and fewer components susceptible to environmental factors.

Track circuits, on the other hand, have higher ongoing maintenance costs . They require frequent and regular maintenance schedules due to their sensitivity to various environmental factors and the need to maintain insulated rail joints. The high level of electrical power and advanced power supply equipment used in track circuits also necessitates more frequent maintenance, further increasing costs over time.

Total Cost of Ownership

When considering the total cost of ownership, an axle counter generally offer a more favourable financial profile. Their low life cycle costs make them an attractive option for railway operators looking to optimise their long-term investments. The combination of lower initial installation costs and reduced maintenance requirements contributes to a lower total cost of ownership over the system’s lifespan.

Track circuits, while still widely used, tend to have a higher total cost of ownership . This is due to the combination of higher initial installation costs and more substantial ongoing maintenance expenses. However, it’s important to note that track circuits may still be preferred in certain situations where their specific capabilities, such as the ability to detect broken rails, are deemed essential.

To illustrate the cost differences, consider the following comparison table:

Cost FactorAxle CountersTrack CircuitsInstallation CostsLowHighLife Cycle Maintenance CostsLowHigh

It’s worth noting that axle counters have been implemented successfully in various applications, offering cost-effective solutions. For example, Frauscher has implemented their axle counter system in Class 1 yards, where track occupation and capacity can be easily determined . This application demonstrates the versatility and cost-effectiveness of axle counters in different railway environments.

While cost is a significant factor, it’s essential to consider other aspects such as reliability, performance in extreme conditions, and specific operational requirements when choosing between axle counters and track circuits. The advantages of axle counters, including high uptime, low power requirements, and overall reliability in extreme weather and environmental conditions , may justify their selection in many cases, despite potentially higher initial costs in some scenarios.

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Conclusion

The comparison between an axle counter system and track circuits sheds light on the evolving landscape of railway technology. Both systems have their strengths and limitations, with an axle counter showing promise in terms of reliability, ease of maintenance, and performance in challenging environments. On the other hand, track circuits maintain their relevance due to their ability to detect broken rails and their fail-safe design.

To wrap up, the choice between these two systems depends on various factors including the specific needs of the railway network, environmental conditions, and cost considerations. As railway technology continues to advance, it’s likely that both systems will see further improvements and refinements. This ongoing development will help to enhance the safety, efficiency, and reliability of train detection systems, ultimately benefiting railway operators and passengers alike.

FAQs

Q: Are axle counters more advantageous compared to track circuits?
A: Axle counters offer several benefits over track circuits, such as quicker and easier installation, reduced maintenance in terms of both time and cost, lower life cycle expenses, higher uptime, minimal power requirements, and dependable performance under extreme weather and environmental conditions.

Q: What are the drawbacks of using axle counters?
A: A notable issue with axle counters is the occurrence of a ‘wheel rock’ failure, where a wheel halts directly above the inductive sensor of the axle counter. This results in the previous section remaining falsely occupied without an actual train present, necessitating a time-consuming reset and restore process.

Q: What are the disadvantages of track circuits?
A: Track circuits can be problematic as they depend on electrical conduction between the train’s wheels and the rail. Rust on the rail surface can inhibit this current flow, preventing the system from detecting the presence of a train, especially in cases of severe rust.

Q: How are axle counters applied in railway systems?
A: Axle counters are increasingly used in modern railway safety signalling systems. Their applications include monitoring berthing tracks in station areas and yards, as well as overseeing point zones in these locations