So what´s the problem? [DC traction stray current control]

The problem of stray currents from DC traction systems is illustrated using a simple arrangement. Current flows from the positive busbar of the rectifier substation, through the overhead contact system, to the vehicle, and returns through the running rails. Because the rails are not very effectively insulated from the Earth, a certain amount of current flows into the earth, especially in the area near the load, and this may find its way into a nearby metallic pipe. Eventually this stray current leaves the pipe and flows through the earth, back into the rails, and finally to the negative busbar of the rectifier substation. At the place where the current leaves the pipe, corrosion may take place and, if the current is large enough, significant damage can occur in a relatively short space of time