The LDSIG rule of thumb recommends the following for curve radii (applies to all scales):
- a minimum radius 3 times the length of the longest piece of rolling stock for reliable tracking. Length is measure from coupling point to coupling point. This minimum radius rule will prevent most all instances of string lining and other derailments from curves that are too tight.
- a minimum radius of 4 times the length of the longest piece of rolling stock improves appearance significantly.
- a minimum radius of 5 times the length of the longest piece of rolling stock allows automatic coupling (but not necessarily uncoupling) to occur.
Note that in N an 80ft passenger car is 6 inches long. If you are running full length passenger cars or other 80ft+ cars or locos, the 3x rule calls for at least an 18" minimum radius.
The rule of thumb assumes body-mounted couplers. Since most, if not all, N RTR rolling stock comes with truck mounted couplers, you can "cheat" to about 2.5 times the longest car/loco length with good tracking reliability except for increased risk of string lining. If you are going to run long trains up grades, then you will have to stay with the 3X rule to avoid string lining.
For turnouts, the NMRA RP (
http://www.nmra.org/standards/rp12_7.html) gives recommended turnout dimensions for various turnouts. Note that these are recommendations only; most turnout manufacturers have their own dimensions. But it's a good guide that will be within 10% or so. Dimension 11 is the closure rail radius, which is the radius of the curved part of the turnout. This should match or be slightly larger than your minimum curve radius for best use of your space with reliable tracking. A #5 turnout shows a closure radius of 13.75" and a #6 has 22".
my thoughts, your choices