Here's an interesting set that I didn't even know existed until it showed up in a collection of mostly Lionel tinplate stuff that I'm handling for our model railroad club (NSMRC). IT's American Flyer HO. Although I converted to HO in 1949, I was unaware of AF HO.
After some research, I found this exact set in the AF catalog for 1938. It is set "HO 100 4 Car Freight Train". The locomotive has an AC motor with spur gears, later replaced by a permag motor with worm/gear drive.
I believe this was the first set that AF made with two-rail track.