Originally Posted by Me

Twin Rails (or Triple Rails, Quad Rails etc)
In an ideal world, we could connect 15 harddrives to a single rail (i.e, molex-cord) and each harddrive would get its due voltage (provided the PSU had a high enough amperage/current rating). We live in reality and due to real-world constraints, having 15 harrdrives on a rail means that (if you manage to boot that is), each harddrive is not getting it's due power. Now a PSU's limitation is not voltage (because those are already predetermined be it 115V, 230V, 12V, 5V etc) but rather it's amperage/current rating (the more current a PSU can deliver, the power power it can provide). Now consider a PSU that can deliver 30A on the 12V rail: you can try and connect a whole whackload of HDDs (say a reasonable number like six) and it would probably boot and most people probably wouldnt encounter problems (or significant ones). So whats the big deal? For power users (or those concerned about system stability or both), the voltage being delivered to those drives may not be 12V but may be closer to say 11V (yes it's a big deal) simply because the PSU has so many demands being placed on a single rail.

Now if we split the 30A into two separate lines, 12V1@15A and 12V2@15A and split the harddrive config to have three HDDs per rail then we could rest easier knowing that the drives are getting their expected/required power. This is the entire point of having multiple rails (two is the most common): to provide another degree of assurance with respect to voltage stability. Devices like high-end video cards require lots of power and sharing that video card with a bunch of other devices is usually not a good idea (even if the amperage/current rating is up to the task) ... a solution would be to use one rail to power the video card and another rail to power everything else.