## 2 Experimental Setup and Tests Conditions  
The setup described in [20] consists in a reduced-scale wheel running on a 1 m full-scale rail. The mechanical laws were considered in order for the scaled-down wheel to be representative of the full-scale behavior of the wheel-rail contact. The aspect ratio of the contact surfaces as well as the average contact pressure at both scales are maintained. The applied normal force on the wheel axis is controlled with an electrical actuator, and the pressure on the contact is dimensioned so as to be equivalent to that produced by a real loaded train. In this paper the sizing of the scaled-down wheel--a roller--was designed to mimic the real wheel of a train of interest used on the French rail network (FRN). For clean, smooth, and fully conductive surfaces subjected to a normal force \(F=400\) N (corresponding to a force of 77 kN/wheel for the real train mimicked), the elliptical contact area half-axes were calculated as \(a=0.47\) mm and \(b=0.29\) mm. The measurement of the wheel-rail electrical contact resistance is performed using the four-point method, as shown in the diagram in Figure 2, which illustrates the principle of this technique applied to the test bench. Two copper thread brushes are used to inject the current from a source into the contact, while two other brushes, placed closest to the contact, are used to measure the contact voltage. The current flowing through the contact is evaluated by measuring the voltage V\({}_{\text{s}}\) across a well-known resistor. The dynamic test series presented in the following Sections 4.1 and 4.2 examine the influence on the contact resistance of the following parameters: degree of oxidation, current intensity, applied normal force, and displacement speed. In the case of oxidation, two main surface conditions were considered. On the one hand, a reference case, known as \"clean\", where a distinction was made between a surface cleaned a few days prior, still shiny, upon which a microscopic oxide layer of unquantifiable thickness had begun to appear, and a surface freshly stripped just before testing, very shiny. These two facies were implemented on 1 m-long rail sections, which can be seen in Figure 3a. On the other hand, a case formed by 'light' oxidations, is intended to describe the situation of tracks with low daily traffic and unfavorable environmental conditions. For these facies, we segmented another 1m rail into three zones, each about 30 cm long (zones L1, L2, and L3), with different levels of oxidation, as illustrated in Figure 3b. These zones have average oxide thicknesses of about 4 \(\upmu\)m, 12 \(\upmu\)m, and 8 \(\upmu\)m, respectively, based on 30 thickness measurements on each (see Table 1). The surfaces were oxidized by vaporizing a 0.6 mol/L aqueous solution of NaCl on the rail. The degree of oxidation was controlled by changing the open-air drying time (respectively 15, 45, and 30 min) at ambient temperature. In the tests, the roller was moved along the rail at controlled speed and normal force. A sinusoidal current of three different amplitudes (\(20\) mA, \(50\) mA, \(100\) mA) at a frequency of 2300 Hz (typical of the UM71 track circuit type) was injected into the contact. For each current intensity, two distinct forces were applied: 200 N and 400 N. For each of these forces, two different speeds were tested: 10 cm/s and 25 cm/s. An incremental encoder mounted on the roller hub enables its position to be known at all times, through a voltage signal \(U_{position}(t)\), and electrical measurements to be synchronized with zones of interest on the rail. The evolution of electrical signals as a function of time is captured using a LeCroy WaveRunner 625 zi oscilloscope associated to a ZD1500 active differential probe (equipment from Teledyne LeCroy, Courtabouf, France).