Interlocking and blocking: the safety apparatus of railways – what it is, its history and types of interlocking

What is interlocking?

Interlocking in railways is a critical safety system designed to prevent conflicting movements of trains and ensure the safe operation of railway networks. It is an essential component of railway signaling that helps manage the complex interactions between trains, tracks, and signals. The primary purpose of interlocking is to avoid collisions and ensure the orderly flow of train traffic on the railway network. 

In the USA, interlocking is officially defined in the railway industry as: “An arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence.

An interlocking system is engineered to ensure that a signal indicating permission to proceed cannot be displayed unless the designated route has been verified as safe. This safety measure is implemented to prevent signals and points/switches from being altered in an incorrect sequence, thereby enhancing the overall safety of railway operations.

Interlocking systems consist of a series of mechanical or electronic devices that control the alignment of track switches and the display of signals. These devices are interconnected in such a way that they prevent conflicting movements, ensuring that trains can only proceed along a route if it is clear and safe to do so. The system relies on a set of logic and rules to enforce safe train movements, taking into account the positions of switches, signals, and the presence of other trains on the tracks.

The key components of an interlocking system include track circuits, signals, point machines (for controlling switches), and a central control mechanism. Track circuits detect the presence of trains on a specific section of track, while signals provide visual indications to train operators regarding the status of the track ahead. Point machines control the alignment of track switches, determining the route a train will take.

Interlocking plays a crucial role in enhancing railway safety by preventing conflicts that could lead to accidents or derailments. It ensures that only one train is allowed on a particular track segment at a time, reducing the risk of collisions and enabling efficient use of railway infrastructure. The sophistication of modern interlocking systems has increased with advancements in technology, incorporating computer-based control systems to enhance reliability and flexibility in managing railway operations.

What is blocking in railways?

Blocking in railways refers to the strategic management of train movements to ensure safe and efficient operations on the rail network. It is a crucial aspect of railway signaling and control systems designed to prevent collisions and manage the flow of trains. The primary purpose of blocking is to maintain a safe distance between trains and regulate their movement in a coordinated manner. In absolute blocking, the track is divided into blocks and the track circuit shows if a block is occupied or free. In old signal boxes, block instruments are used to control the signals. 

Block Instruments Part One An Overview (

With the development of interlocking systems control to electrical relay interlocking, the blocking system is integrated into the interlocking panel which shows the routes set in illuminated tracks that show the position of the trains, and buttons on the panel enable controlling of signal equipment along the track.

Blocking is needed for several reasons:

Safety: The foremost reason for blocking is to ensure the safety of railway operations. By controlling the spacing between trains, the risk of collisions is minimized, and potential accidents are avoided.

Capacity Optimization: Blocking helps optimize the capacity of the railway network by preventing congestion and ensuring that trains are appropriately spaced. This enables a higher volume of trains to operate on the tracks without compromising safety.

Efficiency: Blocking enhances the overall efficiency of railway operations by streamlining the movement of trains. It allows for better coordination of arrivals and departures, reducing delays and improving the reliability of the transportation system.

Resource Utilization: Blocking helps optimize the use of infrastructure and resources, such as tracks, signals, and stations. By preventing conflicts between trains, it allows for the efficient utilization of available resources.

How does the Blocking control system work:

Block Sections: The railway tracks are divided into block sections, which are designated segments of the track. Each block section is equipped with signaling systems that indicate whether the section is occupied by a train. Each block is protected by a signal at the entry and shows the red aspect of the signal to inform the train to stop.

Signals: Signals play a crucial role in blocking by providing visual indications to train operators. Green signals typically indicate that a block section is clear and safe for a train to proceed, while red signals signify that the section is occupied or that there is an obstruction ahead.

Interlocking Systems: Interlocking systems are used to ensure that conflicting routes cannot be set that would lead to unsafe conditions. These systems prevent conflicting movements and contribute to the overall safety of railway operations. Blocking is integrated with the interlocking to effectively control the railways traffic.

Centralized Control: Many modern railway systems have centralized control centers where operators monitor and control train movements across the entire network. Centralized control facilitates efficient blocking and enhances the ability to respond to dynamic operational conditions.

Types of interlocking systems

1. Mechanical Interlocking Systems:

   Mechanical interlocking systems are traditional railway signaling mechanisms that use physical, and mechanical devices to control the movement of trains. The first mechanical interlocking system was invented in Britain In June 1856 by John Saxby for interlocking switches and signals These systems rely on levers, rods, and interlocking components to ensure that conflicting routes cannot be set simultaneously, contributing to the safe and sequential operation of trains on the tracks.

The first mechanical interlocking systems were purely mechanical, however with the need for more advanced signaling and control other systems were used. The other systems implemented were hydropneumatic. In purely mechanical interlocking levers are long enough to give an advantage to move the field point devices directly. With time the signal levers are replaced by small levers or push buttons.

2. Electro-Mechanical Interlocking Systems:

   Electro-mechanical interlocking systems combine mechanical components with electrical elements to enhance the precision and responsiveness of railway signaling. The integration of electrical circuits allows for more sophisticated control over switch points and signals, improving the efficiency and reliability of train movements. In these systems, the levers are smaller and do not move the point devices directly but ensure proper mechanical locking. When the lever is unrestricted in its movement, as determined by the mechanical locking, the contacts on the levers activate the switches and signals. These elements are controlled through electrical or electro-pneumatic mechanisms.

3. Electrical Interlocking Systems or relay route interlocking 

   Electrical interlocking systems rely primarily on electrical circuits and relays to control train movements and ensure safety. These systems use electrically operated switches and signals to manage the routing of trains, offering a more automated and streamlined approach compared to purely mechanical systems.

4. Electronic Interlocking Systems:

   Electronic interlocking systems represent a further evolution in railway signaling, incorporating advanced electronic components. These systems use microprocessors and digital technology to control track switches and signals, allowing for more complex and flexible configurations. Electronic interlocking enhances the adaptability and efficiency of railway operations.

These are also called CBI(computer-based interlocking), Solid State, or Computerized Interlocking Systems. These interlocking systems represent the latest technological advancement in railway signaling. These systems may use electronics and computer-based control to manage train movements with a high level of precision. Computerized interlocking offers advanced features, such as managing automatic timetables, informing the route of a particular train, facilitating solving delays and accidents, efficient remote monitoring and diagnostic capabilities, and contributing to the overall safety and efficiency of modern rail networks.

Evolution of Control centers of railways for traffic control

Early Manual Control:

In the early days of rail transport, traffic control was a manual and decentralized process local to a geographical position. Local operators or signalmen manually controlled switches and signals from signal boxes or long levers along the tracks, leading to limited coordination and efficiency. As railway networks expanded, the need for centralized control became evident.

The first controlling medium was manual long levers that were directly moving the points. The first control systems were the signal boxes that house smaller levers that are connected by wire to signals and by rod to the points. The signal boxes can control several signals and points but locally to a geographical area.

The controlling medium was improved by miniaturizing the levers and transitioning of from mechanical to electromechanical interlocking systems. Finally, the levers are turned into buttons or switches and full electrical relay electronic interlocking.

At the same time, the train detection system(blocking) is changed from direct visual control to the use of track circuits that are supported by illuminated diagrams that show the positions on an interlocking panel.

Telegraph Era and the introduction of CTC :

The introduction of telegraph communication in the mid-19th century marked a significant shift. Centralized Traffic Control (CTC)Centralized traffic control – Wikipedia emerged, allowing operatorsö or dispatchers to remotely control switches and signals over long distances. Telegraphic communication improved coordination but still relied on manual intervention. However, the lever control medium changes to route setting control medium where the route set function is often indicated by diagrams from the controlling signal to the exit signal by white lights ion the diagram on panels. In 1930 as the control became large areas of long lines control centers were set as centers of centralised traffic control center. (CTC). The relay interlocking was fully electrical controlled and hardwires and display panels were the controlling medium of the systems.

However, with the advent of computer systems, the use of hard wires and display panels became a thing of the past and was replaced by computer displays and software systems. Computer technology allowed more management functions that are additional to the signal functions such as train identification and automatic route setting. The first electronic TDM(time division multiplex) systems were used in 1959 in Great Britain for remote control of interlocking. The first computer TDM systems appeared in 1978.

Electro-Mechanical Interlocking Systems:

With the advent of the 20th century, electro-mechanical interlocking systems became prevalent. These systems integrated mechanical components with electrical controls, allowing for more precise and reliable management of signals and switches. Early centralized control centers began to incorporate these advancements.

Computer-Based Control Centers:

The latter half of the 20th century saw a transformative leap with the integration of computer technology into railway traffic control. Computer-Based Control Centers emerged, offering more sophisticated automation, real-time monitoring, and data-driven decision-making. This shift significantly enhanced the efficiency and safety of rail operations.

Advanced Communication and Automation:

In recent decades, railway traffic control has evolved further with the integration of advanced communication technologies, such as satellite positioning and wireless networks. Automation and computerized decision support systems have become integral, enabling predictive maintenance, optimizing train schedules, and enhancing overall network performance.

Integrated Network Management:

Modern Railway Traffic Control Centers are characterized by Integrated Network Management. These centers leverage advanced algorithms, artificial intelligence, and big data analytics to provide a comprehensive view of the entire railway network. They focus not only on real-time operations but also on long-term planning, sustainability, and predictive analysis.

Emergency Response and Security Integration:

Recent developments have also emphasized the integration of emergency response and security measures within control centers. Dedicated Emergency Operations Centers address crises promptly, ensuring the safety and security of both passengers and infrastructure.

Today, Railway Traffic Control Centers continue to evolve, incorporating cutting-edge technologies to meet the demands of high-speed rail, increasing traffic volumes, and the complexities of modern transportation networks. As these centers adapt, they play a critical role in shaping the future of efficient, safe, and sustainable railway operations.