More clues ...
Re: AF circa 1937 ... I think (???) this paragraph refers to (what we now call 1:64) S-scale trains running on 3-rail O-gauge track:
"The 3/16 scale trains were designed to run on O gauge track whose curved sections had 20" radii (formed 40" circles). Importantly, the trains featured fully automatic coupling and uncoupling that were functionally comparable to Lionel's. Unlike Lionel's costly and sophisticated design (each truck contained a solenoid and electrical pickup shoe), the A.F. 'link & pin' (a.k.a. 'harpoon')couplers were gravity based."
Then, AF's post-war flip to 2-rail track ...
"By Summer, 1945 it was able to resume limited manufacturing of the 3/16s scale O gauge trains. While it did so, the same sized products were re-engineered to run on much more realistic two rail (with a "T" profile rail) track. The fine detail of the diecast engines, tenders and cars that had debuted in the '39 catalog reappeared. The engines and tenders continued to be made of diecast metal, but the cars' bodies were made out of plastic. Two pages of the spectacular 1946 catalog emphasize the running advantages of the lighter cars. Ironically, they soon realized that they had to add weight. Metal car bottoms & chassis were necessary to prevent the too-light cars from tipping over. The 'link & pin' automatic couplers that had been introduced on the 3/16s O products were reduced in size, with plastic replacing the sintered metal of the originals."
Based on this (limited) reading, my guess is that Reck's "4-rail" AF track is 3-rail prewar o-gauge, with an extra (4th) rail somehow related to the automatic coupler system.
Anyone out there to confirm this and/or set us straight?
(Quotes via http://www.absoluteastronomy.com/topics/American_Flyer
Or maybe the "4th rail" is some sort of a control system for an "action car", like an automatic dump gondola, some loading/unloading station thing, etc.
OK ... now I'm really going out on a limb!