Charles....
I wrote this very long reply, which I believe could have possibly alienated many a Model Railroader of today but not a Model Railroader of say 1934 in his Attic, Basement or Family Room of 25 feet in width.
So I deleted it.
We call this a Missed Approach in aviation.....
Let me go around again....
Here is the phenomenon......
http://cs.trains.com/mrr/f/88/t/267707.aspx
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So to answer your question in the most succinct manner possible....
It's the one after the 9 o 9 - The Beatles.
Meaning ....
No, not a #8 for a 8 Drivers nor even a re-created #9 Kato Unitrack Turnout from a #6.
I was using an Atlas #10 for 10 drivers and found even that not the best for locomotives of multiple sets of long drivers such as a 2-8-8-2 with an actual train of 30 or more cars behind it.
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Despite what Walter's of Today writes on the Box; their Y6B (2-8-8-2) does not like to pull a train over a #4, but it seems to have no issues with a #12. with 30 cars behind it.
The previous owner of my Y6B, kept sending his back to Walther's...
Three Times and they would pick him out another one...
Never telling him to simply disregard the advertising on the box.
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Even without traction tires on a 1.5% Grade.
My previous standard of 1.73% gave me pause to question.
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I no longer use traction tires..DCC Sound needs all the help it can get.
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So back to the above article....
Do passenger cars derail or uncouple (even in N Scale) as the first writers states on 20, 24 or even 26 inch curves?
Yes  they do on abrupt 90 degree curves with a 21 car passenger train behind three consisted Kato Diesel Locomotives (even though perfectly speed matched).
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iN my reality, N Scale begins to seriously open up at 28 inch curves and even the transition to such a curve shoed be gentle....not abrupt at 90 degrees.
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I may let the world know when I actually create Success On My Layout and write a book which Kalmbach will never publish; nor be allowed in Train Stores.
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Presently, I am experimenting with 28 to 84 inch parabolic curves.
https://www.nmra.org/sites/default/files/d3b3.pdf

But not at the N Scale Level; which was not yet compressed in 1952.
It's the one after the 9 o 9  ...or TT Scale to HO and even O scales which I am currently using; as my basis for Physics.
Lionel O Scale Bridges for example...actually measure out to be N Full Scale.
https://www.youtube.com/watch?v=RES7mnxkYCE
And when one would attempt to actually put an O Scale Ship under such a Lionel Bridge.
https://eaglewingsironcraft.wordpress.com/2012/01/17/o-gauge-vertical-lift-bridge/
Ah?
We are gonna need a smaller boat or get a bigger bridge.
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Or better yet known as BNSF Modern Specifications scaled down to Model Railroading.
And everything else too including the width of your Channel.
What I call Full Scale Model Railroading (without compression)
Meaning ?

Yes, I am in the process of learning the Art of Creating #12 (and larger) Turnouts with Fast Track Jigs.
I am placing these on a Wooden HO Scale Tru-scale Base. and beginning to control then with a ESU like Servo motor back to my ESU ECoS.
https://www.youtube.com/watch?v=4IYvfKwGppo

Question ? >>> Everything 

The Other Phenomenon....
Which gave ma a What the Hey ?!?!
Nearly everyone of my N Scale Brass Steam Locomotives came to me LNIB.
Never run
Even those which had been converted to DCC with Sound.
Here are some clues.....on the proverbial Hallmark 2-10-4
https://www.youtube.com/watch?v=tgwu7ydD4Bw
===> There AIN"T NOTHING WRONG WITH IT !
Just don't expect it to run on 19 inch curves.
When you slop up the drivers so they wiggle to and fro...
Oops !
Then the locomotive may get confused as it tries to traverse a Commensurate Turnout. 
Increase the turnout as well... !
Keep your drivers tight within Box Journals.
Here is another one....
https://www.youtube.com/watch?v=lKP4wSurQ5Y
The Con Car Aero Train.
If you complain to Con For about it...they will send you some weights.
Don't use them....Just go to a wider radius.
Another example is the Kato Shinkansen using Single Viaduct Kato Track....
Won't work..
http://www.jnsforum.com/community/topic/4376-kato-single-track-viaduct-connect-to-viaduct-station/

Kato's Double Viaduct Track has a clearance of 15 mm rather than 10....and I increased even these dimensions when it came to my #9 Kato Turnout.
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Again as at the first article strongly suggests...
It's the one after what ever the Manufacturer Suggests is where you should begin your track laying efforts.
Today...
There isn't much on my layout that I have not modified from the instant, I took it out of the box.
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Except for Lok/More N Scale Steam Locomotives (The best the hobby has ever offered).
I've got to get me one of these.....
http://spookshow.net/loco/more4664.html

And will so, as my reward for creating a layout which can actually run it.
Sorry Spookshow...
19 inch curves ARE THE PROBLEM.
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I learned as an Aviation Pilot....
Not to simply read and believe the specifications as set forth in written form.
Instead....
Take the Airplane out on your own, and see what you can personally make it do.
Then if you actually do loose an engine on take-off.....
Meaning....
===> I won't fly a Cessna 310 into most small airports.
The roll out procedure on one engine out on take off...exceeds the length of most small airport runways.
They are fine with both engines operating...but is you loose an engine....It will be considered as Pilot Failure to investigate the minimums. and the Insurance may not kick in.
Such as ...
https://www.aviationlawmonitor.com/2010/02/articles/general-aviation/tesla-cessna-310-crash-at-east-palo-alto-the-paradox-of-the-twin/

Yes, this airport and the one North of it too at San Carlos.
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So a 4-6-6-4 works easier than say a 4-6-2? on the same given trackage?
>>> No 
The Kato 4-6-2 is a phenomenon unto itself.and I strongly suspect a whole other layout will be required to effectively operate a LIK 4-6-6-4.
:)) Mark