Ralph says:

Hi Djon;
In answer to your query about rail sizes;—- The number in the size listing ie:(code 100), (code 55) and so on, is the weight of the rail per yard long.
Different size rail is used for the varying load requirements for the locomotive tonnage and weights of the loaded cars. The”Main Line” also refered to sometimes as the “High Iron” will be the heaviest weight needed to cary the combined loading of loco and freight car tonnage that is used over a particular route between destinations the goods and other freight being transported are traveling. Smaller sizes such as code 55 may be all that’s needed for sidings or branch lines etc.; again depending on tonnage necessities of the line. Smaller locos etc. don’t by themselves require the heavy weight rail, and the smaller rail is less costly for the Railroad’s budget.
So to complete your answer as far as modeling; using different size rail, the larger size rail looks very disprportionet to the size of the locomotives and cars as compared to say code 80 or code 55. Look at some photos of the newer smaller size rail as compared to the older larger scale rail and you’ll see what I mean. Hope this helps you starting out on your modeling. I might add that the more fussy you are with your trackwork the better your equipment will run. Constant derailments are no fun and can cause one to loose interest in the hobby quickly due to the frustations of these problems. If you e-mail me I’ll try and answer any other questions you may have.
[email protected] Good luck—- Ralph