How do you know which rail you are connecting stuff to?
By looking at the wires with the yellow color coded insulation. These are the +12 Volt lines.
The solid yellow insulator jacket is usually considered to be the primary +12V rail.
If the yellow wire also has a different colored stripe (e.g. blue) this means it's on a different rail.
On power supplies with multiple +12V rails each rail has a different colored stripe on the yellow insulator jacket.