# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
###### Abstract  
Railway signalling systems must ensure the safe movement of trains and the reliability of the operation of their components is of utmost importance. One of the main components is the interlocking devices, which provide secure, safe and reliable interaction between points and signals in the controlled railway station area, and the safe movement of trains in this area depends on their proper functioning. In this article, the failures of the signalling devices in the Bulgarian railways over a three-year period (2020-2022) are analysed and processed according to a developed methodology. First, a statistical assessment of device failures is performed, comparing the number and duration of failures of different types of equipment, and calculating proportional ratios. Second, a reliability analysis is carried out and the reliability indicators are determined--mean time between failures _MTBF_, intensity of failure flow, availability and unavailability coefficients. The obtained results clearly demonstrate the need for determination of the complex reliability indicators. They give the clearest assessment of the state of the devices. If only a statistical assessment of failures and their duration is made or if only simple reliability indicators are calculated, erroneous conclusions can be drawn regarding maintenance and the need for modernization. a 2025 10 March 2025  
tracking and predictive maintenance is proposed to improve rail safety and efficiency. According to [3], it is necessary to modernise signalling systems and introduce new safety techniques due to the events and mishaps that occurred on the railway network and the growth in traffic. The theoretical principles for the safety train traffic control system synthesis is presented in [4], considering the capabilities of equipping infrastructure facilities with highly reliable and safe means of technical diagnostics and monitoring. An advanced train traffic control system architecture is developed in [5], which is based on the railway infrastructure facilities monitoring techniques integrated into the railway automation and remote-control equipment. Another cause of serious accidents is discussed in [6]--this is the increased speed of the train, which can lead to abnormal interaction between the train and the control system. It is proposed to continuously monitor the operation of the systems during train's movement in order to check the correctness of its own behaviour while generating control commands. A method of close headway analytical estimation when building railway safety management systems is presented in [7]. Analytical estimates of train spacings under railway safety and security terms with speed limits on the running line and minimal spacing between trains departing from and arriving at the station are provided, as the influence of the radio channel on the close headway is taken into account. The advantages of new communication technologies and their application in railway transport are also commented on in [8], but it is emphasised that very strict safety requirements must be met. New methods for monitoring infrastructure facilities are being developed. In [9], a comprehensive detection program for space-air-vehicle-ground network integration based on unmanned aerial vehicles (UAVs) is presented. The methods of fastener safety status and foreign object detection are proposed, the railway safety level of the large-scale road network can be improved, and a platform support for a comprehensive railway safety detection system can be provided. Another method that will assist in preventing numerous rail accidents is developed in [10]--Railway Track Tracer technology for Crack Detection. This is a technology that detects cracks in railway tracks using machine learning. In [11], an Internet of Things (IOT)-based railway safety monitoring application is proposed to provide real-time assessment of the state of safety and efficiency in railroad bridges, tunnels and the welfare of residence areas surrounding the railroad infrastructure.