Maybe you’ve been asking, “What size rail should I use for the track on my HO gauge layout?” Code 100 and code 83 are the sizes most readily available in HO gauge. Code 70 is a smaller size which is also readily available but not so common. In N gauge, code 55 would correspond to code 100 in HO gauge.
First let me give a little background information: The code number gives the actual height of the rail in thousandths of an inch. Code 100 is correspondingly 0.100 of an inch high, code 83 is 0.083 inch high, code 70 is 0.070 inch high, and code 55 is 0.055 inch high. This is actual inches, not scale inches.
How then does this correspond with prototypical rail? Code 100 would correspond to rail that weighs 156 lbs. per yard. That is very heavy rail, and prototypically was used only by the Pennsylvania Railroad and then only for a short period of time in western Pennsylvania where they ran heavy coal trains. Model railroaders complain about code 100 rail looking too large to look real with some justification.
Code 83 rail corresponds to prototypical rail weighing 132 lbs. per yard. That is very common on mainline and even on branch lines today. In the last two years they put down 136 lb. welded rail on the Dakota, Minnesota, and Eastern Railroad (aka Canadian Pacific now) through my home town of Dodge Center, MN, and 132 lb. welded rail in Claremont, MN, the next town to the west. It replaced very old (1920’s) jointed rail that was only 90 lbs. in weight. In contrast to today, 132 lb. rail was used only on the busiest mainlines in the 1950s. What size model rail you choose may depend somewhat on the era you want to model.
Code 70 rail would correspond to 100 lb. prototypical rail, and code 55 in HO gauge to 75 lb. prototypical rail. 100 lb. rail was commonly used on mainlines around World War I, while 75 lb. rail was used in the 1800s.
There are other factors besides realism that may affect you choice of track rail. Will the flanges of your rolling stock run on the smaller rail without derailing? Particularly if you have plastic wheels, this will be an issue. Model railroaders I know would encourage the use of metal wheels for this reason and for other reasons, such as greater ease in keeping the track clean.
Other factors may include cost. Code 100 generally is the least expensive with the most product options, though code 83 is close behind. Code 100 is also more rugged. The model railroad club I belong chose code 100 for their layout because it would hold up better when it became necessary to climb onto the layout for construction purposes or for operational problems that may occur in the “back 40.” I use code 100 on my layout simply because I had a heavy investment in that size track when I began. I am also finding out that spikes, ballast, and debris along the track is less of a problem due to the greater height on the larger size rail. My model railroading friend in town uses code 83. The cost to him was only slightly higher and the track he obtained had more realistically sized and colored ties in addition to the more realistically sized rails.