Question

What are Single rail and Multi rail PSU?

Answer 1

A "rail" or voltage rail refers to a single voltage provided by the PSU (short for power supply unit). An ATX PSU has one 3.3 volt rail. It also has one 5 volt rail. The 3.3 volt rail has its own circuitry in the PSU which generates the voltage. 5 volt rail has its own circuitry. similarity PSU has one 12 volt rail however now two or more 12 volt rails also being packed in PSUs. So the rails are the circuitry of voltage level generation in a PSU. Single rail PSU will have one tye of voltage output and multi rail have multi voltage outputs. In your case single rail stands for one 12 volt output having its own circuitry and connectors and dual rail have two 12 volt outputs having their own circuitry and connectors apart from each other.