Verified answer

The rail is basically the transformer that converts the wall power to DC power for the computer to use. Multi-rail is generally more economical. It consists of many rails and so all the power supply connections connect into one of these rails. The problem with multi-rail is that power needs to be balanced, that is one rail should not be fully loaded while another is almost unused. That is not a big issue and the connections usually come so that the load is balanced. A single rail supply is mainly known for its flexibility. Because it is just one big rail, there is no balancing needed. Although single rails can be considered somewhat better, multi-rails are much more cost-efficient (generally) and it really won't make a difference especially if the computer is not necessarily top of the line power hungry. I hope this helps and I have given some articles in the sources.