I have a 2-8-0 that works on 9-3/4″ R and 12″ R. These are the radii of my test track. After watching all my loco’s and cars go around the 9-3/4 loop I don’t like the way it looks. Just too sharp to look at on a continuous basis.

When I have a 2 inch to 2 1/4 inch oc center in the straight a ways, I will expand that to 2 3/8 to 2 1/2 on center in the curves. This allows for overhang of your engines and especially long ones and cars such as auto racks or passenger cars. It also allows the center of a car to clear where it insets during the curve. You have to allow for swing and inset in order to make sure there is clearance. These I use for HO. Just find the equivalent in N Scale and that may take some setting of temporary track to do the same and measure that. Standard snap track curves also put your wheel trucks suddenly into curves and what you should really do is transition to the radius you want. Each track entering the curve should slowly convert to the needed radius and the tracks with the larger radius should start after the first so as to increase the radius and get the curve clearance at the same time. Some modelers tend to separate their tracks with enough space to be able to handle their car or engine without disturbing the other track which in reality most dual tracks are relative close. I think only out west where there is vast space for trackage rights do they really have lots of space to do that.
As far as radius the shallower the better for most trains and engines. Long trains also don’t like tight turns. If your engine is suppose to have a minimum radius, It will usually work but with lots of overhang in the curves and you are straining its limits as the wheel trucks hit their frame stops and any jog in the rail can have an effect on it’s performance. Find space to have a realistic radius and give the engine the chance to perform as it should. Hope this helps…..
from Newman Atkinson Terre Haute Indiana