Curved double track spacing

I have yet to lay any of the 2nd track of my planned double track sections, some of which go around some curves which readers here will
probably regard as quite tight. My layout is N scale. My actual innermost track (already laid) has a radius of 315mm on the tightest curves.
I've mainly been designing and building to the European NEM standards because they're more comprehensive and comprehensible compared to the NMRA standrds, and also because I'm keener on European and Japanese prototypes than USAian ones.
But I think I have found what seems to be an error in the NEM specifications for the radial distance between two curved tracks. And as a result I don't know what the correct figure is for the minimum spacing.
The most obvious place to start is NEM103 `Umgrenzung des lichten Raumes bei Gleisfhrung im Bogen' (`Dimensions of the structure gauge for curved track') http://www.miba.de/morop/nem103-d.pdf . The table on p2 show that for class C rolling stock the value of the parameter `E' is 5mm. The total swept width is B1 + 2 x E where B1 is 27mm (from NEM102 http://www.miba.de/morop/nem102-d.pdf ). So that would give a minimum spacing of 37mm. For a straight track (NEM102) the width and thus the spacing is just B1, ie 27mm.
But NEM112 `Gleisabstaende' (`Track spacing') http://www.miba.de/morop/nem112-d.pdf gives different values. Firstly, the track spacing for straight track (para 2) is quoted as 25mm. This is 2mm less than the NEM102 figure. Secondly, the table on p2 gives a spacing of 31mm (for the next radius down, 300mm), which is fully 6mm less than that derived from NEM103.
These discrepancies are quite substantial. What, if anything, am I missing ? Surely the track spacing should be derived from the dynamic envelope in exactly the same manner as the structure gauge.
I looked at the NMRA specs too. S-7 `Clearances' http://www.nmra.org/standards/sandrp/pdf/S-7%202008.01.pdf suggests a clearance of 30.2mm on straight track. S-8 `Track centers' http://www.nmra.org/standards/sandrp/pdf/s-8.pdf says that for a 12.5" (317.5mm) curve, for Class I stock, I need 1+5/16" or 33.3mm.
This brings me on to another question. The last time I bought a loco I found it quite difficult to find out its minimum requirements for track geometry. Is there some conventional way of finding this out without asking the retailer ?
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Ian Jackson personal email: < snipped-for-privacy@chiark.greenend.org.uk>
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Ian Jackson wrote:

That's not too bad. Most common commercial smallest radius is around 250mm.

"Correct" is a chimera. If you really want to be "correct", find out the dimensions for your prototype and your time frame, and convert those. Otherwise, compromise, like we all do. Even NEM.
IOW, don't sweat it. ;-)
[snip discussion of NEM data]
I've read your plaint about the NEM stuff, and I'm not surprised. Almost everything you quote is (or should be) a Recommended Practice, and not a Standard; or else Prototype Data. Very few areas of modelling need to be standards, so it's not really surprising that the bits you quote don't mesh - they don't have to.
If you are running long modern rolling stock, go with wider track spacing. If you are running late 19th/early 20th century trains with short locos and cars, go with narrower spacing.
Also make sure have good easements leading into the curves. Not only looks better, but you'll have much smoother operation.
HTH
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Wolf Kirchmeir

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Best bet is probably to take the largest item you have and use that to mark the curve, using the outboard corner of this vehicle on the inner track. If you then repeat this with a temporary outer track using the centre point of the vehicle on the inside of the curve, and the lines cross, you need more space. A carpenters flat pencil is handy for this.
Just a thought.
Mike
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Mike Smith wrote: [...]

Yes, but a good thought. ;-)
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Oh, I can certainly do that. But what if next year a really exciting model comes out which needs more clearance ?
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"Ian Jackson" wrote

Same problem can arise on the 'real railway'. If a train doesn't 'fit' it's not allowed to work that route.
John.
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That's the mainline. I'm expecting to build various 230mm minimum radius branch lines, and a yard, but they won't take all of the stock. I'm looking at 230mm because that's the curve radius of Peco's sharpest points.
315mm seems to be the minimum design radius for Kato's models of various modern high speed train prototypes. I have a Kato Shinkansen Nozomi 500 which I'm very pleased with.

*snort*
Certainly in a layout in which I plan to fit the maximum amount of track, with several levels and some quite severe gradients, inside a 1600x1000mm glass-topped table, things aren't going to be very close to the prototype.

...
My worry is that if I build a bunch of track to a certain spec now, I'll find later that the model I really want (probably not even manufactured yet) won't run on it. I was sort of hoping that the NEM would give me some guideance which would be useful to avoid that pitfall.

I don't really have room for proper transition curves. But things seem to run well enough anyway; when I do have trouble it seems to be alleviated by cleaning the track (and occasionally the wheels).
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wrote:

You are relying on virtual transitions which is a concept not unknown to the full size railway. A two axle vehicle is not fully on a given radius until both axles are on the curve. Up to that point the vehicle is making its own virtual transition. The same is true of a bogie vehicle.
George
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Well, that is of course true but I think it isn't really what people mean when they talk about the need for transition curves.
Nevertheless I shall have to remember it for if I'm ever challenged by a purist. Mind you purists will probably be too horrified by the very entertaining GIANT MUTANT COWS (OO-scale on my N scale layout) to start worrying about the details of curve radii.
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