The railroad frog
"A one-piece cast frog. The shiny line crosses the rusty line. This is an example of North American "self-guarding cast manganese" frog, where guard rails are not used, the raised flanges on the frog bearing on the face of the wheel as it passes through the frog." Quote from Wikipedia
N scale is small to me.Maybe your wheels are too large and ride up and over?I guess it depend on the brands you are using.