A dual rail is a 12V power supply voltage divided between two sub-rails. During the splitting of the individual voltage rails, the voltage remains at 12V, although the current is divided between the two sub-rails.
DC-DC (direct current to direct current) is an innovative technology used by power supply manufacturers. Conventional power supplies use a transformer to generate the 3.3V, 5V and 12V current. With DC-DC technology, only the 12V current need be generated in the transformer. Potential transformers are used to create the other two voltage rails.