Although rail size can vary. How do you know you're comparing the same size of rail in scale?
(For example, "standard" HO track uses "Code 100" rail (the code is the height of the rail in thousandths of an inch). Code 83 is the next most common for HO. I've been handlaying Code 70 and Code 55 (yes that's half the size of the Code 100 found in most sets). Common N scale rail sizes are Code 80 (massively over-scale) and Code 55 (similar to Code 100 in HO, which is also over-scale for most rail).)
I recently sent a 2 pack of Cobra Head Street Lights to a guy in Canada and he said they were a little big for HO. I was told by others that they were nice lights and 4 inches was about right for city street lights. I also compared them to lights I saw in a hobby shop and they were the same size. Is 4 inches right? Frank's HO SCALE LIGHTING and BILLBOARDS at http://www.hoscalelighting.com
I know this an older post, but I had to get my 2c in. As I understand, HOn3 (commonly called 'narrow gauge') refers to the width between the rails and therefore the engines (and rolling stock) that can ride on them. HOn3 refers to a 3 foot (or 36 inch) width between rails, while HOn30 implies 30 inches between rails. An On3 would be 3 SCALE feet between the rails.
As it turns out, O narrow gauge can use standard HO track, and HO narrow gauge can use standard N gauge track.
BTW, great scale comparison photos! Wish I had seen them when I was younger. That distinction drove me nuts as a kid.