The specific NMRA page on track centers is here.
N scale is toward the bottom of the page.
Layout Design GalleryLayout Design Special Interest Group
Welcome to the forums The curve radius on a 36 inch wide layout is actually pretty good for N gauge. It's the equivelant of 30" or more for HO.
Springfield PA
Welcome to the forums.
Look on the NMRA web site under 'Resources', then 'Standards and Conformance'.
http://www.nmra.com/
You should be able to find everything you want to know about spacing and such.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
I am planning a small (36" by 80") layout in N scale. Obviously the curve radii have to be small. I haven't been able to find any information on track separation for parallel track, both straight and curved.