### Quick Reminder of the Properties of Wheel-Rail Electrical Contacts  
The works concerning the electrical properties of wheel-rail contact are less numerous than those related to adhesion problems, for example. In this section, we present a brief state of the art on this subject. Usually, in the railway field, without considering the technological evolutions (e.g., weight reduction, etc.) that can also deteriorate the quality of the wheel-rail electrical contact, two types of factors may be distinguished that have an influence on it:  
* The presence of what is called a \"third body\" between the two elements in contact. * The existence of an oxidised layer. The third body, present between the solid surfaces in contact, is mainly composed of particles detached from them (the wheel and the rail) and of contaminants (such as dead leaves, water, fragments of ballast stones, sand, oil, various pollutants, etc. ), with a thickness from a few micrometres to several tens of micrometres on the rail and wheel [9]. The study of the role of the third body is particularly complex due to the permanent changes in the strongly related geometric and electrical properties of this interface material throughout the movement of the train [10]. [PERSON] et al. studied the influence of the compactness and heterogeneity of the third body on the shunting ability, and they observed that the characteristic of the third body that leads to poor shunting seems to be its heterogeneity. [PERSON] et al. [11] conducted an experimental study to investigate the effect of sand-blasting on the electrical insulation of a wheel-rail contact. Indeed, the presence of sand can compromise the quality of the contact. Static and dynamic tests were performed. They observed a transition in the contact resistance according to the quantity of sand between the two elements. They proposed a model of electrical insulation by assuming either a total separation of the two metallic parts by a layer of sand, or a partial contact of the disc with a certain amount of sand. They also observed [12], for instance, that the larger the grain size of the sand, the less it affected the degradation of the wheel-rail contact. Concerning problems related to surface oxidation, several studies have been conducted as early as 1960 [13; 14; 15; 16] to better understand the shunting phenomena in the case of a stationary or moving train. These studies were carried out either at scale one or at smaller scales (e.g., cylinders rolling on one another or in a ball/plane configuration) to more precisely study the effect of an oxide layer on the contact, for example. From an electrical point of view, the injected signals corresponded either to those of the track circuit or to \"laboratory\" signals. The studies focused on two parameters: the contact resistance and the breakdown voltage (i.e., the voltage sufficient to allow the current to flow between the wheel and the rail). In the case of shunting at standstill on scale-one devices, and also on laboratory devices at a smaller scale (e.g., two cylinders rolling over one another), the studies showed that beyond a certain voltage the contact resistance sharply decreases. This voltage depends on various parameters, such as the time of application of the voltage, the contact pressure, and the degree of hygrometry. Moreover, the contact resistance and the breakdown voltage do not seem to be correlated a priori. The main results can be summarised as follows:  
* The breakdown voltage depends on the contact pressure, the environmental conditions (oxidation, pollution), the contact surface, and the speed of movement. * The measured contact resistance may show an irreversible change, which would correspond to a change of state in the contact. * Time constants have been found in the evolution of the contact resistance and the breakdown voltage. Indeed, it was observed that the current in an oxidised contact can only flow when the oxide layer is \"cracked\", since even a very thin oxide layer is sufficiently resistive to create a high resistance.