## 1 Introduction  
Train detection systems play an important role in the rail signaling systems. They provide information about train positions in the railway infrastructure, which is crucial for managing the traffic in a safe way. Apart from the basic information about the track section unoccupancy, the system may provide other valuable information such as, e.g., speed of the moving trains. Highly accurate and safe information about the actual speed of a train is very important for control over a train movement. Starting from the train driver who needs to know the current speed of the vehicle, through the automatic train protection systems (ATP), which monitor the permitted speed and if it is exceeded activate braking, up to a fully automatic train operation (ATO), which not only slows down the train if the speed is too high, but allows to operate the train in a fully automatic, driverless mode. For such purposes, the speed is measured by tachometers [1,2] that are parts of the train equipment. In [3] the authors discussed different methods of speed estimation based on the encoders installed on the axles of vehicles. The method based on optical technology [4], Global Positioning System (GPS) [5] or Eddy-Current sensors [6] can also be used, but they all involve vehicle equipment. Information about the speed is then available for the driver, the onboard systems and, in the case of ERTMS (European Rail Traffic Management System), also for the RBC (Radio Block Centre) thanks to the GMS-R communication (GMS for Railways) [7]. The information about the train speed may be also very useful for other systems that use axle counters but have no access to the information provided by vehicle equipment. The systems which may use the information about the train speed are for example: a level crossing--operator notification system (class A level crossing system), an automatic level crossing system and an information system for passengers waiting at the platforms. The data about the train speed may be also used as diagnostic information, especially for monitoring the condition of the tracks [8; 9] and for the purpose of performing the End-of-Train detection function. The standardization of rail traffic control systems in Europe includes the train speed as a one piece of information to be provided by axle counter systems. The initiative is led by the EULYNX consortium, established by the leading European Infrastructure Managers (inter alia: DB Netz, Pro Rail, SNCF, Infrabel, Network Rail and many others). The current available standard for the interface between the Train Detection Systems (TDS), including axle counters and the interlocking systems, the main interfaced system for TDS, i.e., the SCI-TDS Specification doc. SCI-TDS EuLoc.44 v3.3 (Set 4) specifies the message with information about the train speed and a wheel diameter. Today, there are no safety requirements imposed by EULYNX for providing such information (Basic Safety according to CENELEC EN 50129 [10]), however it may be expected that it will change in the future, if such information is used for purposes other than just for diagnostic ones. In the case of the first two applications (Level Crossing systems), the speed of the train may be used to delay the barrier closing. In [11], the authors present an issue of barriers closed for a long time being a bottleneck for the traffic on the level crossing and propose a solution based on the continuous vehicle and infrastructure communication. A more dangerous situation happens when the people waiting next to the closed level crossing try to circumvent the barriers and cross the tracks. Lack of a train for a long time (and even 2 min may be a long time for waiting people) results in irrational human behavior, leading to dangerous situations. According to Polish Railway Infrastructure Manager (PKP PLK S.A.) ca.