After the current passes through the locomotive, from the pantograph on the roof down to the wheels and further to one of the rails, it must return to the converter station. At regular intervals, the electricity is then conducted from the rail up to the return conductor that runs along the contact line.

By having the return conductor close to the contact line, electromagnetic interference with, for example, the telecommunication cables along the railway is avoided. The tricky part is getting the electricity to prefer going up into the return conductor rather than staying in the rail. Therefore, suction transformers are placed along the railway.

The suction transformer ensures that the current from the rail is drawn up into the return conductor. It strives to pull back as much current in the secondary winding as passes through the primary winding on its way forward. This can be compared to a vacuum, thus forcing the current from the rail up to the return conductor.

Since the late 1990s, a more modern system has been introduced that allows for equally low levels of interference as with suction transformers. The new system instead uses autotransformers and can therefore transfer a larger amount of current (power).