S-scale has its roots in the American Flyer brand. Before World War II, A.C. Gilbert bought the American Flyer product line, and made trains that could operate on the Lionel's O-scale three-rail track. The equipment was made to 1:64 scale, but the wheels and trucks were gauged to the wider O-scale standard. After WWII, to be able to better compete with rival Lionel, A.C. Gilbert developed the more prototypical-looking two-rail system, and spaced the rail gauge to that of S-scale. Lionel, in its O-scale track, uses a third rail down the center of the track, since it primarily used AC power for the engines. A.C. Gilbert won the battle of which of the two had the most realistic-looking track.
There are four distinct groups within the S-scale community. All model in S-scale, or the 1:64 ratio, but with some slightly different flavors.
Typically abbreviated as "A.F." or "AF", sometimes also called "tin plate" especially when referring to pre-war items.
Because of S-scale's American Flyer roots, there are a lot of people who still have and perhaps operate original A.C. Gilbert American Flyer products. Lionel now owns the "American Flyer" brand name, and they continue to manufacture products under that name. These are compatible with the older A.C. Gilbert A.F. products.
The significant difference with A.F. is that this style of modeling uses the larger knuckle coupler, deep-flanged wheels, and the wheel tread is wider. The couplers are incompatible with Kadee-style couplers used by scale modelers (but can generally be swapped-out fairly easily). The AF deep-flanged and wider wheels will, generally, run on straight or curved track, if the rail height is tall enough. However, due to the wider width of AF wheels, they will not operate on "scale" turnouts. However, "scale" wheels should operate just fine on AF or "high-rail" turnouts. "Scale" wheels and trucks are available and can be used to replace AF trucks, if so desired. AF modelers usually also enjoy the A.C. Gilbert style of accessories, such as animated structures and signs.
Officially, for S-scale, the NMRA defines "deep-flange" or "hi-rail" modeling as any equipment or track needing a rail height of code 125 or taller (i.e. 1/8" or taller rail). Generally, modelers who would flag themselves as hi-rail modelers are those who used to be American Flyer modelers, but are slowly transitioning or incorporating more to-scale features in their models and layouts.
"Scale" modeling typically refers to more-to-scale modeling. This covers both standard-gauge and narrow-gauge track gauges. This style of modeling is identical to what you would see in N- or HO-scale modeling.
There is also a small group of S-scale modelers who use the Proto:64 standard for track and wheels. This standard uses an exact down-sizing of the real track dimensions. This requires very accurate track components and track-laying disciplines, and matching exact-size wheels on both locomotives and cars. There are a few companies that make Proto:64 parts. If done with the required accuracy, this will yield very realistic-looking track and wheels.
S has Sn3, Sn2, and Sn42, representing, respectively, 36", 24", and 42" rail-head spacing. These designations are also found in nearly all of the other modeling scales.
Modeling in narrow-gauge in S-scale is a lot easier than in the smaller scales. The engines track better. There is more space inside for DCC, sound, and/or battery power. The details are easier to see. P-B-L is the main Sn3 equipment manufacturer. Sn2 is primarily supported by Portland Locomotive Works. Bar Mills, the structure kit manufacturer, has an Sn2 layout in the foyer of their business office, built by the owner Art Fahie and some of his friends. There are a few people who model in Sn42 (a.k.a. 3-1/2' gauge). Railmaster Exports is one manufacturer who provides locomotive kits (both steam and diesel) for this gauge in S. These are primarily intended for New Zealand, South Africa, and Canada modeling of 3-1/2'-gauge prototypes.
The modeling style is an individual choice; whatever gives you the greatest enjoyment out of this hobby. After all, that is the objective of a hobby! All "flavors" of S-scale modeling are actively supported within the S community and by the NASG and on this web site. The "united" word in the NASG logo refers to the fact that we are all still 1:64-scale modelers.
As a side note, lest you think these issues are unique to S-scale only, N-scale used Rapido couplers that are incompatible with the Micro-Trains or Kadee-style couplers, as well as deep-flanged wheels; HO-scale used to have the old horn-hook couplers that are also incompatible with Kadee-style couplers; O-scale is very similar to S-scale, in that it, too, has claw-style couplers that are incompatible with Kadee-style O-scale couplers and deep-flanged wheels. Additionally, O-scale has the issue with their rail gauge being five feet wide, not the correct 4' 8-1/2", and they use a center rail to conduct electricity.
So, if you are interested in running any kind of American Flyer type of equipment, look for A.C. Gilbert items (pre-1969), or look for AF-compatible or "hi-rail" in the product description. If you are a scale modeler, look for newer products, and order them for "scale" or find out if the AF-compatible product can be converted to scale.
Currently, about 60-70% of the S-scale community fits within the A.F./hi-rail group, and about 30-40% are scale modelers. Outside of the track, wheels, and couplers, there is, of course, a tremendous overlap of other items such as structures and scenery items.
The whole purpose of the main Resources button is to help find what is available to the S-scale modeler. Armed with the knowledge from the paragraphs above, you will be better able to figure out what style of modeling you would enjoy and which products you want to purchase to build your layout.