The 8x16 HO Scale Layout as seen in the picture above has at best 36 inch radius curves.
Yet, I bought with it on the very same day, an HO Scale Brass Cab Forward, manufactured around 1965, the same time as John Allen't Open Letter.
Done the Math yet?
I do not yet know what it will take or need into a curve but I suspect at least a 44 inch radius and perhaps as wide as 56 inches; with an actual train behind it.
Meaning...The above layout began to fall apart as I moved it...No matter, I was to rebuild it anyway with much wider curves.
The whole purpose of a layout is to run Cab Forwards.
For I saw one once in 1955, though the widow of my Parents Black and Pink Buick; with Port Holes on the side....> I hit the automatic transmission lever on the column as I crawled up to press my face against the window...Oops! My father never could get it to run right after that, and we soon traded it in on a 1957 Chevy with Tail Fins !