Fairly elementary question, but I don't know it because we've only ever used one track code and I don't remember what it was when we first purchased it. What's the difference between the different track "codes", 70, 83, 100 etc? I know it's in the size of the rail but which way do they go, from smallest to largest? Which is the most accurate for prototypical mainline rail, lightweight siding and branchline rail, etc?