The current enters the train via the pantograph (current collector, i.e. the bow between the train and the contact wire or overhead wire), leaves the train via the wheels and then finds its way back to the substations via the rails. This is called return current. Part of this return current runs through the earth. In a direct current system, this is called leakage current (or stray currents, electrical currents that drift through the soil). 

In a alternating current system, the rails are earthed, so it is logical that part of the return current runs through the earth. 

Because a current always seeks the line of least resistance, part of the current may leak out of the alternating current system and run into the adjoining rails of the direct current system, and vice versa. The part of the leakage current that runs into other installations, is called interference voltage. Interference voltage may influence the functioning of these other installations, for instance the railway safety installation in existing trains (ATB, the system for automatic train control, both the equipment on trains and on the track).

Besides, the alternating current system causes electromagnetic fields that may interfere with telecom equipment and sensitive equipment of others, such as hospital equipment. To prevent disturbances in telecom equipment along the existing track, copper cables along the track are replaced with fibre optic cables.

All conductors needed for the AT system are suspended in a specific manner relative to each other. As a result, the electromagnetic fields are kept to a minimum, while the greatest part of the return current runs through the conductors (i.e. the rails and the so-called return current conductors). This means that the interference voltage becomes so slight that it does not cause interferences in other systems.

On existing track, which the high speed trains use before entering a station, in the station itself, and when leaving a station, the so-called ATB (automatic train control system) prevents a train from neglecting a red signal. In the event a train passes a red signal, ATB automatically applies the brakes.

If interference voltages interfere with ATB, the railway signals may jump to red haphazardly, and trains may come to a standstill unexpectedly, which affects the availability. But a signal may also erroneously jump to green, which puts safety at stake.

To prevent disturbances as a result of interference voltage in ATB, a small part of the ATB equipment in existing trains has been replaced. In addition, the safety detection system, which indicates whether a train is on a specific section of the track, must be adjusted.