DC Electrified Rail Transit Systems:

Electrified rail transit systems are found in numerous major cities, such as New York, Chicago and Toronto. Stray current corrosion problems can arise in DC traction systems in which the rails act as the primary current return path. For practical purposes, it is virtually impossible to insulate these rails completely from the ground. The ground surrounding the rails and any metallic structures buried in it represent a conductor that can carry stray current. Reportedly, corrosion damage was attributed to streetcar return currents as early as 1891.

In some countries, buried structures are still directly bonded to the rails. This practice is generally outdated and undesirable, as a large amount of "uncontrolled" stray current can enter the ground and affect surrounding infrastructure.

Schematic illustration of stray currents in DC electrified transit system.
Click on image to enlarge

Note that there is a fundamental tendency for the return current in the rails to flow from the train to the substation. At the same time, because the track is not perfectly insulated from the ground, there is a tendency for current to flow in the ground (the ground essentially represents a conductor parallel to the tracks) in the same direction. In the simplified scenario depicted above, stray current will tend to be picked up by the pipeline (which represents a low resistance current path within the ground) near the train and discharged near the substation.