# Enhancing Railway Network Safety by Reproducing Wheel-Rail Electrical Contact on a Laboratory Scale
## 4 Conclusions  
In this study, we designed a scaled-down rolling contact test bench that is able to mimic the wheel rail electrical contact of a train at the laboratory scale. The design was based on mechanical considerations, and the criteria applied on the test bench can effectively reproduce wheel-rail electrical contact. The bench was coupled with an original homemade salt spray system designed for obtaining controlled rail oxidation. Through a series of carefully controlled tests, including variations in load and oxidation state, we validated that the electrical behaviors observed were coherent with the observations from previous works. In a second trial, the behavior of the wheel-rail contact in static and rolling conditions was studied in order to better understand the deshunting phenomenon. In static conditions, the influence of contact force and current intensity was successively investigated. With regard to force, contact loading-unloading tests were carried out with previously polished surfaces and a DC current of 0.1 A. The resistance vs. force curves, plotted in log scale, show different patterns for loading and unloading: as force increases, resistance first decreases slightly, then more markedly; on return, resistance increases with an intermediate slope. We assume that this behavior, which is in most cases far from the Hertzian contact model, reflects the influence of the surface roughness remaining after polishing (measured peak-peak values of around 4 um on the roller and 8 um on the rail), with plastic deformation of asperities occurring between 200 and 300 N. With regard to current intensity, we compared non-oxidized and oxidized rail surfaces for a given wheel state and a contact force of 400 N, with the current being increased and then decreased in  
Figure 17: Detailed view over a few periods showing voltage drop evolution across (blue curve) the wheel-rail rolling contact and (red curve) a known shunt resistance (6.8 \(\Omega\)) during rolling contact under 400 N normal load on rusted wheel–rail contact elements. steps between 0.1 and 1 A. In the case of \"clean contact\", the \(U(l)\) characteristics are found to be linear and reversible. In the case of an \"oxidized contact\", the initial resistance is much higher, and the increase in current is accompanied by several sudden decreases in resistance, reflecting an irreversible evolution that we interpret as the flattening of asperities softened by heating due to high local current densities. Long-term observations of the evolution of contact resistance at 1 A current seem to corroborate this explanation. On return, as the current decreases, the \(U(l)\) characteristics are almost linear, with a much smaller slope (resistance). This set of results in static shows that the experimental bench we have built allows to study how wheel-rail contact resistance may be affected by mechanical, thermal, chemical or electrochemical phenomena. In rolling conditions, the very first feasibility tests, carried out under real-life conditions except speed (slower), revealed alternating zones of shunting and deshunting, with different possible deformations of the contact voltage signal compared to the sinusoidal imposed current signal for the latter. Such behaviors were observed in previous studies both on a reduced-scale laboratory bench and in 1:1 railway tests. For further study, we intend to deepen our investigations by focusing on dynamic and AC tests in order to reproduce a wider variety of real-life railway situations and explore more fully the influence of key parameters on the deshunting phenomenon. Conceptualization, F.L., F.H. and P.T. ; methodology, L.H., F.L., F.H. and P.T. ; software, L.H. and F.L. ; validation, F.L., F.H. and P.T. ; formal analysis, L.H., F.L., F.H. and P.T. ; investigation, L.H., F.L., F.H. and P.T. ; resources, T.C. and F.G.; data curation, L.H. ; writing--original draft preparation, L.H., F.L., F.H. and P.T. ; writing--review and editing, F.L., F.H., T.C., F.G. and P.T. ; visualization, L.H., F.L., F.H., T.C., F.G. and P.T.