Daytime chat has been discussing this some... The facts (this comes from Nov 2000 MRR): In 1921 mainline rail averaged 83 lbs/yd which is equivelent to HO code 55. In 1956 mainline rail averaged 105 lbs/yd which is equivelent to HO code 70. By 1983 mainline rail averaged 117 lbs/yd which is equivelent to HO code 83. The delima: Since even lighter rail is common off the mainline, this may make it difficult to model with the correct scale rail everywhere on your layout. Compounding the problem, is the fact that some manufacturers don't make all the weights of rail you may desire. So, what can be done to build a model empire without upsetting the rivet counters, hand laying all of your track, working little plastic people to death, and /or scratch building your own rail??? The options (the opinions?): The first option might turn some rivet counters green, but you could just ignore the code and go with what works, is available and/or cheapest. As Shamus says, the chances are noone will notice the difference once it's all ballasted and weathered. Here code 100 would have the advantage that everyone seems to make it, so you can use cheap flex track with higher end turnouts. Code 83 would be nice if you can get the turnouts you want. The next option would be to go with heavier rail on your mail line (or branch), then use a lighter rail in the yards. This may allow more realism while maintaining manageable track that "runs anything". You may also get away with cheap flex track this way, especially if you can go 100/83. I also recall some discussion about how what's scale and what looks right isn't always the same thing. I think we were talkin' tele pole spacing or something, but to me code 100 doesn't look wrong. I look at the little spur by my house and it looks a lot like code 100. In any event, it seems wise to run something darned reliable in out of sight and hard to reach areas, like tunnels and the back of the layout. Thots?