In a reversing loop, if the rail polarities match at one end, they are opposite to the other. It is therefore necessary to be able to reverse the polarity in order to ensure the concordance at the desired location, that is to say where the train passes. It can be deduced therefrom immediately that the train should not be longer than the loop itself, at least if the tail vehicle has axles capable of bridging the rail cuts.
We can see that this inversion requires four contacts, two normally closed and two normally open. When the KA8 relay is not energized, the loop zone is normally powered, as shown in the diagram: the polarity matching is correct downwards. When the BOUCLE route is requested, relay KA8 is energized and then the rails polarity in the loop area is reversed: the match is now correct upwards.