Do turnouts somehow coincide with track radii????

Model Railroader General Discussion (Model Railroader)
1

That is, does a #6 turnout (with appropriate small curved piece) correlate to an 18" radius,

a #8 for 22" radius and # 4 for 15’ or what?

thanks

2

Nope, #bered turnouts have no relation to radaii as they have a straight diverging route. If you use Atlas Snap Switches or European Peco switches, they have curved diverging routes.

David B

3

The N. American style, as David says with the diverging route straightening at the frog and beyond, has what is known as a substitution radius. You can find what that is at the NMRA site where they discuss turnouts. The substitution radius for a #6 is something like 42", quite a bit. But you can only begin to follow your proper curve after the frog if you are willing to clip the rail stubs on the turnout. It isn’t advisable, but some folks have resorted to doing that out of desperation. You are far further ahead to use a curved turnout where both routes are curved.

4

Here is a link to the NMRA Recommended Practice table of dimensions for HO turnouts

www.nmra.org/standards/sandrp/rp12_3.html

Note that there are TWO radii associated with the RECOMMENDED dimensions of each numbered turnout. The first, and tightest, is the radius of the closure rail between the heel of the point and the straight rail through the frog. (This is the one that will put your 4-12-2 on the ground when you drop a low # frog turnout into what appears to be a wide curve.) The second is the substitution radius, the one the turnout, with its lengths of straight rail, will simply drop into without kinks if you cut a pre-laid curve and plop a turnout into the hole.

Now, two jokers - the first being that manufacturers do NOT produce turnouts to exact NMRA dimensions. Atlas ‘#4’ turnouts are actually #4.5. You actually have to measure and test-lay pre-built turnouts to find out if they will work. (Bummer![|(])

The second joker is better. By trimming the length of straight rail AFTER the frog, you can drastically reduce the substitution radius. Prebuilt turnouts typically have considerable lengths of straight rail. If you trim the rail on the curved leg to the shortest length that will still accept a rail joiner (for a metal frog) or to the insulated frog (Atlas) and lay the curve from there, the substitution radius comes WAY down - but still not as tight as the closure rail radius.

Right about here I would normally put in a pitch for hand-laid turnouts, but I realize that isn’t everyone’s cup of o-cha. For the curious, simply enter hand-lai