Code 83 rail is slightly lower in height. It is .83" high and code 100 rail is .100" high. I have went with code 83 rail for mainline and yard tracks as it is the most prototypical size for those purposes, and I will be using code 70 rail for back tracks n lightly used spurs. All my track so far is Atlas code 83, including all my #6 n #8 switches and it works great! I sent you a PM by the way. Check out my thread in the layout planning section if you like under 2'x15' yard ideas...