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Discussion Starter #1 (Edited)
I'm in the design stages of track plans for either an Sn3; or, On30 layout. I plan to lay all track, for whichever scale I decide to go with. The NMRA gives some information on track radius, although the information given is for 4 feet 8 and 1/2 inch standard gauge. NMRA RP-11, in their Track Dimension Table on page three, Gives some Degrees of Curvature for curves for all scales. I'm interested in designing a layout that could be used for either On30; or, Sn3. I think that a 30 inch radius might work for either scale. In O scale a 30 inch radius translates to 118 foot radius, or, 50 Degrees of Curvature. In S scale a 30 inch radius translates to 158 foot radius and 36.5 Degrees of Curvature.

I'm thinking that both are really pretty tight curves. Maybe not so bad for Sn3: but, really tight for On30. However, since On30 is really sort of a mythical gauge, there might be some, what I'm going to call flexibility; or, ease in plausibility!

Do you have any thoughts on this matter? If so, do you model in either On30 or Sn3?
 

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Depends alot on size of equipment your going to run. I have On30 shays on logging line running on 15" radius HO track with no trouble.
 

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The idea is to create something notional, believable, but mostly 'functional,' in a defined space. The engineering that limits your practical enjoyment and operation is set by the people who import the locomotive(s) and rolling stock you hope to use. So, go by that information. Rough out a track plan, mate the parameters up with the specs stated for a range of possible engines and rolling stock, adjust the curve radii where necessary, and then construct your plan. IOW, it's sort of a iterative process until you are happy with all of the specifications inherent in all aspects of the plan you have in mind. From there, ideally with a scaled diagramme, you get materials and necessary tools, and start construction.

It's by no means necessary, especially if you have a scale drawing of your track system, but I have always profited from laying out the track system schematically on the floor of the space I'm going to use. I use masking tape to lay it out, whether on concrete, tiles, or carpeting, so that I can see the confines of the benchwork, how the curves will work to make the track flow, and so on. Standing over it, a job that typically takes me close to an hour, I can see where I should make adjustments, or add or subtract trackage. Maybe a siding should be longer, or it could be, for example.
 

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Discussion Starter #4
I have decided that I might lack room for either an On30 or Sn3 layout and am looking at HOn3, now.
 

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We rode the White Pass and Yukon Railway and they had one curve so tight the engine was behind you on the other side of the valley. A quick google didn't show what the radius was but maybe someone knows.
 

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On30 and HOn3 both have quite a bit of industry support these days, so you should be able to find a fair amount of equipment at reasonable prices. We have quite a following for HOn3 (myself included) here in Colorado because there are still 3' narrow gauge passenger lines in operation today.

One advantage with a smaller scale, you could run two trains in opposite directions around the room and have space for passing sidings. A little work with an arduino and some IR LEDs could make it so the first train that reaches the pass tucks into the siding and waits for the other train to go by before proceeding. Things like that could add a little more interest to what would otherwise be just a simple loop around the room.
 
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