i am currently building a new n scale layout and limited on space. i cant go any larger than 12 " radius is this too tight for operation or will it be ok. and if it is ok how tight can i go without having problems?
12-inch radius can work pretty well with all but the longest N-scale equipment. So a lot depends on the era and types of cars and engines you will be using.
Fred Wright (who posts on this forum) and members of the Layout Design SIG developed a good rule-of-thumb guide to rolling stock length versus radius.
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I'm not an N guy, but short steam and 4 axle diesels should work OK. I know Bachmann says the minimum for things like their Dash 8, 6 axle locos is 18".
I have a lot of small and medium sized locos and rolling stock and have no trouble on 11 in radii, I do not recommend running the bigger modern diesels, nor any steamer bigger than a ten wheeler to be realistic... Though just between us I have a bachman spectrum 2-6-6-2 that ran just fine pulling a monster of a consist just to check out my plan...