There is a great debate about how to qualify S gauge trains. "AF" or "American Flyer" can be the more toy-like trains that A.C. Gilbert produced until 1965 or the reruns that Lionel is producing under the American Flyer name. Lionel has two segments. One segment they call "traditional" that includes engines and rolling stock very similar to A.C. Gilbert but with better rolling characteristics, and another segment they call "scale" that includes very highly detailed engines and a cylindrical hopper. The Lionel "scale" engines come with the large toy-like couplers and wheels with deep flanges that work best with rail larger than would be prototypical. The engines do allow a user to change the huge couplers for more scale-sized couplers such as the Kadee 802 (black), Kadee 808 (brown), or MTH scale couplers at the user's expense. There is the option of buying scale wheelsets for the diesels, but the steam locomotives require a big expense to change from what are "high-rail" wheels to scale wheels. The Lionel hoppers are nice, but they sit too high on either scale or high-rail wheels. (It's a long and sad story. There are details on this forum. A search on "post-mortem" can bring up the pitiful details.)
There is another camp known as "high-rail" that uses the taller rail with deeper flanged wheels. The deeper flanges reduce the chances of derailing on less-than-perfect track and can also accommodate the older A.C. Gilbert models. The high-rail camp can be a gray area in that some rolling stock can be extremely well detailed with scale couplers added at a little extra expense or can also be well-detailed models with the large couplers for compatibility with American Flyer engines and cars. This in-between stage is a choice some modelers make who want to run both the old American Flyer models, the latest American Flyer models from Lionel, or the well-detailed models from American Models, MTH, and S Scale America. For AM and MTH products a user must specify "high-rail" or "scale" to get the correct wheels; the bodies of the cars are the same well-detailed models. S Scale America rolling stock models sold by Des Plaines Hobbies and some distributors include both high-rail and scale wheels sets (excepting the stack well car for which a buyer must specify the size). For all three suppliers the trucks allow the swapping of high-rail or scale wheelsets fairly easily in all cases except the two American Models steam locomotives. All of these models have superb rolling characteristics with either type of wheel and the appropriate track.
The scale camp tries to maintain prototypical accuracy with smaller rail, smaller and realistically flanged wheels, and very precise details. Scale locomotives and rolling stock can generally run on high-rail track, but there are only a few limited switches that will allow them to pass through without derailing. High-rail models or the "traditional" models have the same problem with scale switches--they derail at the frog point. They also often have problems with the flanges being so deep that the flanges run on the ties and bounce along scale track.
If details are important, the Lionel American Flyer Legacy locomotives are excellent. The same is true for any of the American Models, MTH, or S Scale America products. The latter three give you the choice of high-rail or scale wheels. In all cases you have the choice of buying your own scale sized couplers for more prototypical appearance. You may also have to buy coupler pads to put under rolling stock trucks to keep the height that would be missing once you remove the large American Flyer style couplers (often called "lobster claws").
S Gauge produce has come a long way since A.C. Gilbert. Today's products can often be confused with far more expensive brass models. They are larger than H.O. and allow a lot of detail to be added by a modeler without straining old eyes. They are smaller than O gauge and allow a lot more scenery and track in small spaces. If you decide to go the S gauge route, the chief decision is to decide whether you want to go the traditional American Flyer route with deeply flanged wheels and oversized couplers, go the scale route for the highest modeling accuracy, or make a compromise with the higher rail, deeper wheel flanges, and scale sized couplers. Whichever decision you make, you'll find the S gauge community to very welcoming and helpful. (Maybe 'cause we're smaller.)