Article: A few lines of the Paris Metro in France operate on a four-rail power scheme because they run on rubber tyres which run on a pair of narrow roadways made of steel and, in some places, concrete. Since the tyres do not conduct the return current, the two guide rails provided outside the running 'roadways' double up as conductor rails, so at least electrically it is a four-rail scheme. One of the guide rails is bonded to the return conventional running rails situated inside the roadway so a single polarity supply is required. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey. The loop was originally provided to save the original steam locomotives having to 'run around' the rest of the train saving much time. Today, the driver does not have to change ends at termini provided with such a loop, but the time saving is not so significant as it takes almost as long to drive round the loop as it does to change ends. Many of the original loops have been lost as lines were extended.

Now answer this question: What is required in order for the guide rails to operate properly?
a single polarity supply