Railway Interlocking Systems

Railway signalling systems are very critical systems. Any dangerous situation which may occur in the system can cause very dangerous accidents. Therefore, interlocking systems are used to prevent any hazardous cases in the signalling systems. Interlocking mechanism described under the previous topic is implemented to the signalling equipment and it is called the railway interlocking system.

Interlocking is the core system in railway signalling. It ensures that all signalling equipment are in proper status for train movement. Basically, it obtains information about train occupancy and locks the movable wayside elements in correct position for a certain route. Then, it permits movements via signals.

Depending on the technological developments, different kind of interlocking systems are developed until today. The first developed system is the mechanical interlocking. Almost every element were mechanical equipment in the first interlocking system. Movable elements were being controlled by steel wires and there was not any train detection mechanism. Signalling operator who stays in a control tower at the station area checks the presence of the trains, sets the points sequentially and clears the signal by mechanical levers. Interlocking of the wayside equipment is achieved by a device called locking bed (Figure 1). It only permits safe possible state combination of the wayside equipment.

Figure 1. The locking bed mechanism

Electro-mechanical interlocking systems are developed in the end of century. The central interlocking unit was still a mechanical device but wayside elements was being controlled by electrical or pneumatic actuators.

The next technology used to developing interlocking system was relay based technology. In that technology, mechanical interlocking mechanisms leaved their objects to the complex relay based interlocking circuits. They were also called “all-electric” signal boxes. Route setting was achieved by selecting start and target signal on the control panel (Figure 2). This technique was the first used entrance-exit (NX) method to set a route.

Figure 2. A relay interlocking system and a control panel

The next step was the development of systems with electronic component in the 1980s. The fact that the logic is implemented by software rather than hard-wired circuits in electronic interlocking technology. Modern monitors were used to manage the system instead of old NX panels.

In United Kingdom, the first generation microprocessor-based interlocking called Solid State Interlocking (SSI) is developed. It was the brand new developed technology before the Computer Based Interlocking (CBI) systems.

Nowadays, one of the new trends is to develop interlocking systems which based on PLC devices. Through new developed safe PLC devices, it is possible to develop safe, reliable and flexible PLC based interlocking systems. In this thesis study, an approach to develop PLC based interlocking mechanism is represented.

Reference: M. Bellek, "Design And Rams Analysis Of Railway Interlocking Based On Formal Methods: An Example Application", Master thesis, TU Dresden, Dresden, 2013.