The flow of electric current through the axle box roller bearing of railway vehicles is one major problems for the safety and reliability of rolling stock. The electrification of networks, the increase of power installed on board vehicules and higher speeds lead to an increase of risks of bearings damaged by electric current flow. As a result, the use of Earth Return Current Units, already indispensable on electric locomotives, is being extended to other vehicules by the large European railway companies. Since 50 years, Mersen devices by virtue of their effectiveness (low contact resistance, permanence of contact), are very well suited for the protection of such bearings. On all the vehicles of a train, the currents that can flow through the axle box roller bearing have various origins and their magnitude varies from the first to the last vehicles. The origins are : - The electric locomotive or power car : Traction current in the return current path and current supplied in the return path from trailer vehicles. This latter origin also applies to diesel electric traction. - The trailer vehicules equipped with generators supplying power to several trailors of a train where the return path is formed by earthing of the vehicules. - The track : return current of trains running on the same track; signalling currents; stray currents emanating from industrial trackside installations. In the 3 cases above, train and track are parallel conductors. - Accidental : faulty insulation on one or more pieces of electrical equipment on board any of the vehicles; overhead line falling onto the train. The flow of current through roller bearings wilst the vehicles are stationary or running, produces damages characterized by craters or scoring (washboarding) respectively and whose development is dangerous. This result in substantial oversheatings leading to the destruction of the roller bearing or seizure of the axles. To avoid such damages, it is necessary to use devices shunting the greatest part of the current through the axles and wheels without flowing through the roller bearings. Some references : Metro : Los Angeles LACMTA (US) SMRT (Singapore) Cairo (EG) Tramway : Toronto, Portland, St Jose, Bonn, Rotterdam, Los Angeles, Melbourne