Why is single rail preferred? Can anyone explain in detail? Most 500-650W PSU's are dual rail.
Two rails mean that each one only can provide half of the PSU's total power. Good chance you'll overload it just putting a normal size load on it from miners, as all the PCI-e connectors would only have half of the total wattage to use.
It is quite cheaper to build multi rail jobbers, as the transformer and voltage regulation can use much less Robust (less expensive) parts.
Just in weight a good P/S will have about 5lbs of copper in it.