I was looking at the trams and wondering how the current gets back to the substation. I know that the current flows out along the overhead cables and then down into the trams and into the electric motors. But for there to be a complete circuit, it has to flow back to the substation. The answer is that it flows back through the wheels and the rails. Why doesn't that hurt people? Because it is at very low potential. It is apparently a problem that the current will return via the Earth by going into utility pipes and causing corrosion by electrolysis
http://www.tautonline.com/stray-current-myth-or-legend/
