I'm looking to due a n scale passanger train layout, my question is what is the minimum curve radius i can use?
Typical n-scale cars representing 1930's to present passenger coaches are about 6 inches in length. My experience with such cars suggests that operation on 13 3/4 inch radius is okay, but smaller radii may lead to problems. This is based on testing a variety of brands (Walthers, Micro-Trains, Intermountain, Kato & Con-Cor). My opinion on appearance is that anything less than about 18.75 - 19 inch radius seems inadequate.
Seventeen inch and above is best. Fifteen inch is passable. Thirteen inch is the minimum. Some of them will run even on nine and three quarter, but will look pretty bad. In more than a few cases, full size N scale passenger cars will derail on nine and three quarter curves and maybe even eleven inch.