In practical terms, the bigger the turnout number, the longer the railcars you can run through them without problems.
As a teen, I learned this when I was trying to run my Athearn blue box SD45 through #4 turnouts and found out it didn't like them at all. We didn't have as much choice back then, but I pretty much have stuck with #6 since then as a minimum. Oh, I model 70's and 80's so yes, 6 axles diesels and long freight cars are included.
Rio Grande. The Action Road - Focus 1977-1983
Has anyone done an actual, detailed study of what the model manufacturers offer as #4 or #5 or #whatever turnouts? I mean with micrometers and laser calibrators and whatnot. NMRA RP-122 notwithstanding, it seems that some store-bought turnouts use the same #4 frog but increase the radius of the diverging track and call the new thing a #6. Or a #6 a #8. Or whatever.
Peco has eschewed numbering altogether and calls their turnouts small, medium, or large. Perfectly reasonable and descriptive, but some folks might want exactitude.
I'd be curious to see if such a study or report exists.
Robert
LINK to SNSR Blog
The number refers to the frog angle. The #4 frog (and it's extension except on a curved turnout) diverges 1' every 4' (or 1" every 4") and is a more severe angle. The #5 diverges 1' every 5'.
There are other differences in the other parts of the turnout. A key one is the curvature of the point rails and closure rails (between the point rails and frog). On the #4 they will be sharper.
You can find extensive detail in the NMRA turnout dimensions Recommended Practices area and other related pages:
http://www.nmra.org/rp-122-turnout-dimensions
In practical terms, a #4 is pretty sharp. Some locos and long rolling stock (like 80' cars) may not take it. A #5 is much more forgiving. I used #5s in my yards, #6 for mainline takeoffs and one #8 for a smoother mainline split.
Paul
Modeling HO with a transition era UP bent
One. [rimshot]
But seriously, folks... Turnout number refers to the amount of spread of the rails coming off the frog. The number is the ratio of the distance from the frog to the distance the rails spread apart. On a #4 turnout, the rails are 1 unit (inch, cm, scale foot, whatever) apart 4 units (inch, cm, scale foot, etc.) from the frog. On a #5, they're 1 unit apart 5 units from the frog. So smaller numbers are sharper. Typical numbers that commercially offered turnouts come in are #4, #5, #6 and #8. These are much sharper than on the prototype, where #12 is considered very sharp and #20 is more typical.
--Steven Otte, Model Railroader senior associate editorsotte@kalmbach.com
What is the difference between a #4 turnout and a #5 turnout?