Many model train sets today use only two rails, usually associated with Z, N, HO or G-Gauge systems. These are typically powered by direct current (DC) where the voltage and polarity of the current controls the speed and direction of the DC motor in the train. A growing exception is Digital Command Control (DCC), where bi-polar DC is delivered to the rails at a constant voltage, along with digital signals that are decoded within the locomotive. The bi-polar DC carries digital information to indicate the command and the locomotive that is being commanded, even when multiple locomotives are present on the same track. The aforementioned Lionel O-Gauge system remains popular today as well with its three rail track and AC power implementation.