# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 2 Research Methods  
The purpose of signalling systems is to ensure safe movements of trains on a railway infrastructure by locking movable track elements in a proper position, checking the clearance of track sections, locking out conflicting moves, and controlling train movements in a way to keep them safely apart [26]. The interlocking devices (IID) are one of the main components of signalling systems and railway infrastructure. These internal devices provide secure, safe, and reliable interaction between points and signals in the area of the controlled railway station [26; 27; 28]. The interlocking area may contain tracks protected by controlled signals on which trains may originate, terminate, pass, and turn. These are the station tracks, and their arrangement forms the station area. The term \"interlocking\" comes from the fact that the signals governing train moves are interlocked with movable track elements (points, driven by point machines, also called switch machines) and signals governing conflicting routes. In the interlocking area, the controlled signals are interlocked with points and other signals. Therefore, interlocking is based on the principle that a safe route up to the authority limit is established, and this route must meet the following conditions [26]:  
* All points must be properly set and locked and must be kept locked as long as the train is allowed to run on them. * Conflicting routes must be locked out. * The route must be protected against unintended movements on converging tracks. * All sections of the track through which the train is allowed to pass must be clear. The local interlocking devices consist of interlocking systems and user interfaces for the on-duty dispatcher. Contemporary interlocking stations also allow remote control from a main dispatcher centre. In Bulgarian railways, the National Railway Infrastructure Company (NRIC) is responsible for the construction, development, repair, maintenance, and operation of the railway infrastructure. Specifically, regarding safety systems (including interlocking devices), there are three units (parts of the NRIC), called \"Signalling and Telecommunications\" Departments (STD) in Sofia, Plovdiv, and Gorna Oryahovitsa (G.Or. ), which are responsible for the activities on the operation and maintenance of the devices in the respective area. There are various types of interlocking devices in operation on Bulgarian railways that can be classified into the following main groups [28]: route-computer interlocking devices (RCID), route-relay interlocking devices (RRID), electromechanical interlocking devices (EMID), relay systems for key dependences (RSKD). The number of interlocking devices, systematised by type and location, is given in Table 1 (data are taken from the Reference documents of the railway network, issued by NRIC in the end of each year--links to publicly archived datasets are provided in the Data Availability Statement Section). Traffic lights control consoles are also still in operation--temporary (used during construction activities in renovated stations) or permanent (at stations without signalling installation), but their number is very small, and they are not the subject of this study (links to publicly archived datasets). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of IID** & **STD—Sofia** & **STD—Plovdiv** & **STD—G.Or.** & **Total** \\ \hline RCID & 3 & 20 & - & 23 \\ RRID & 62 & 53 & 54 & 169 \\ EMID & 17 & - & 15 & 32 \\ RSKD & 25 & 20 & 22 & 67 \\ Total & 107 & 93 & 91 & 291 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of interlocking devices (\(N_{\mathit{ID}}\)) in the different STD. Each of the three STDs continuously collects data on failures of devices of signalling systems--not only of interlocking devices, but also of the following:  
* external station devices (ESD)--point machines, rail current circuits, counter points, traffic lights, etc.