Electrical behaviour of the wheel-rail contact

In railway signalling systems, a shunting principle is frequently used to detect the presence and position of a train on a track section. In some situations, the detection can become difficult essentially because of the electrical behaviour of the wheel-rail contact. This situation is called a shunting malfunction problem. To be able to fit rolling stock and/or the track circuits, the only knowledge of the various responses of the track circuit voltage receptor does not suffice and a more robust understanding of physical phenomena involved has become necessary. This paper focuses on two experiments dedicated to the study of the electrical wheel-rail contact. The first experiment concerns measurements on a reduced scale bench in laboratory, whereas the second aims to investigate the same parameters in the context of a real site with an instrumented train on a portion of commercially used track. In both cases, static contact and rolling contact have been studied, however only the rolling contact, which is clearly the worse case, will be considered in the paper. Time transient current-voltage characteristics have been measured and analyzed varying some main parameters such as voltage and current range, signal frequency, applied load. The results obtained on the reduced scale bench prove to be in good agreement with those from site tests. A contact voltage saturation phenomenon was observed, which is discussed in the light of recent papers relative to Branly effect.