Code 100 is what has been marketed the longest....years and years. Nowadays, many still prefer it because it is robust, inexpensive, and generally in good supply. But...in scale, it amounts to 170 pound rail, and no one in N. America ever used such a weight. Only the Pennsy and some other roads used 155 pound rail. So, as stated earlier, Code 83 is gaining more favour as we go along. However, Code 70 is probably closer to what you can expect on a main line where the freights aren't especially fast or heavy, and now Code 70 is becoming the 'weight' of choice for many modelers.
-Crandell