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Let's clear up any confusion about the term "rail". A voltage "rail" refers to a single voltage provided by the PSU (short for power supply - actually it stands 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. It also has a lot of wires and connectors to distribute the 3.3 volts to any hardware which needs it. The 5 volt rail has its own separate set of circuitry, wires, and connectors to deliver 5 volts. Modern ATX12V PSUs may have as many as four separate 12 volt rails. Each 12 volt rail has its own set of wires and connectors just like the 3.3 and 5 volt rails. The 12 volt rails just happen to be generating the same voltage as each other.
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