sizzling :
Unless you know the actual design you have to take the spec on face value and that's 18A per rail (per PCIe cable)
A rail does not = a pcie cable. That's what you are misunderstanding. There are other components of the computer where the GPU gets power besides the PCIe cable. For instance, the main 24-pin ATX cable. As seen with the RX 480, after the fix, it should get about 70W from that rail, leaving maybe 80-120W from the PCIe cable. Not only is that all far within the rail's specification the "spec" of the rails don't mean much and
shouldn't be taken at face value. The only thing multiple rails is related to is overcurrent protection, and the trigger points for overcurrent protection are usually set very high, above rated amperage. But either way he won't even get near the trigger points so it won't be an issue.
Rails are just designations for different 12V wires, or usually cables.