## 2 State of the Art
The test procedure involves verifying the conformity of application software and installation correctness with the design and with the requirements of the infrastructure administrator. Such tests are conducted in cases in which a setting command is feasible and will be correctly executed by the system (satisfied command execution conditions) and in cases when a setting command should be rejected by the system (unmet command execution conditions). Unlike certification tests, these tests do not involve safety studies. Tests after the modernization of on-post equipment and devices modernized at adjacent posts and tangent lines are conducted to confirm the correct equipment cooperation and proper interface operation. A further group of tests is those executed within the operation process as part of periodic diagnostic testing. This involves checking all functions implemented within the devices in terms of their availability. The checks are conducted in the course of implementing a diagnostics plan pursuant to all relevant diagnostics and overhaul manuals [6; 7]. In the course of implementing tasks at a traffic post, some system functions are executed very rarely or not at all and cannot be included in the diagnostic program for the post. In summary, functional tests are conducted:* Within the process of device certification and assessing devices newly put into operation;
* During acceptance of devices and linkage systems (commissioning), whether: -- Newly constructed; -- Retrofitted; -- After reconstruction (e.g., station track system, adding new device functions); -- Temporarily decommissioned, prior to restoring to operation. * Within the operation process: -- Cyclically, at fixed time intervals, pursuant to OMMs; -- After identifying disturbed device operation; -- After removing failures that require repairs, replacing damaged elements, and adjustments; -- After railway accidents, within railway accident committee investigations. Each of the aforementioned cases entails the execution of a specific set of tests. Depending on the equipment's functional configuration and test objective, the set can differ in terms of test cases. It should be stressed that the tests are conducted on active devices that form the basis of traffic management, and tracks are closed off for the test duration, with a part of the track system excluded from traffic. It is also necessary to involve the block post-testing personnel to operate the devices (setting routes, setting block system direction, etc.). These operations result in reducing the availability of traffic control devices. Moreover, they influence the block post-traffic situation. Hence, they may lead to train traffic disturbance. Here, managing train traffic and following a timetable are a priority; therefore, test execution is impacted and the tests have to be suspended for the duration of a scheduled train passage. The fact that task execution--route setting, train passage, and route release--is also a test checking the configuration status of the implemented route is a positive aspect. Validation is a crucial aspect of a system's life cycle sequence. The validation stage includes a comprehensive system inspection in terms of conformity with the documentation and requirement specification. According to [8], validation activities are required to check the system against the requirements and assumptions and to confirm its security. Sources from the literature [9; 10; 11; 12] directly refer to system validation as a basic life cycle stage for security critical systems to prove that the requirements are met. They describe the testing essence at the development stage employing various methods recommended by normative documents, as well as proposed expansions and modifications of the recommended methods. The authors of [11] proposed an expansion of the FTA (fault tree analysis) method to FTTD (fault tree with time dependence) and an analysis of the THR (tolerated threat) method to analyse the probability of a catastrophic failure based on stationary Markov processes.