There's not that much difference between single rail and multi rail PSUs. The whole current carrying capacity of the multi rail PSU is available even w/o all the +12V cables being connected. It basically boils down to what it will take to shut down the PSU as it gets loaded up. On a multi rail PSU, if one rail's current exceeds the rating for that printed circuit board 'trace', it will shut down the PSU. On a single rail PSU, there is no Over Current Protection (OCP) per trace; the current would have to exceed the single rail rating to shut down the PSU. Both PSUs still provide the same amount of power as needed.
See this for a better explanation: http://www.overclock.net/t/761202/single-rail-vs-multi-rail-explained