rack rails, dragsters, and tracks used in railroads the term rail is used in reference to power supplies, though its usage does come from our common sense understanding of the word. A power supply rail is in some devices (especially older ones) literally a metal rail that passes through the device to deliver power to the components. A voltage is applied to the rail from the power supply and various components are connected to it for their power. Usually there is more than one rail, and each one delivers a different voltage and has different components tied to it. If you crack open an old power amp or mixer you stand a pretty good chance of seeing a couple. In most modern and/or low power devices there aren't any actual rails, but the concept lives on. When you hear the phrase 'power supply rail' what is being communicated is some type of bus with a set voltage applied to it for the sake of powering components in the device. The connection may be delivered by wires or traces on a circuit board.">Aside from rack rails, dragsters, and tracks used in railroads the term rail is used in reference to power supplies, though its usage does come from our common sense understanding of the word. A power supply rail is in some devices (especially older ones) literally a metal rail that passes through the device to deliver power to the components. A voltage is applied to the rail from the power supply and various components are connected to it for their power. Usually there is more than one rail, and each one delivers a different voltage and has different components tied to it. If you crack open an old power amp or mixer you stand a pretty good chance of seeing a couple. In most modern and/or low power devices there aren't any actual rails, but the concept lives on. When you hear the phrase 'power supply rail' what is being communicated is some type of bus with a set voltage applied to it for the sake of powering components in the device. The connection may be delivered by wires or traces on a circuitboard.