Question about Atlas Tutnouts & Numbers

Model Railroader General Discussion (Model Railroader)
1

I am going to re-do a portion of my layout and need some info about turnouts. On the present layout I had several turnouts that came with a section of track that will turn into an 18’’ Rad. curve, what is the number of this turnout? I do not want ANY 18’’ curves and will be making as many as possible 22’’ and bigger. When Atlas says a # 6 turnout is it a sharper turnout than a # 8 or the other way around? Is the bigger the number the less angle the turnout?

Thanks, Mike

2

Yes

3

The one your talking about is called a “snap switch” It doesn’t have a #. If your running short 4 axle locos and short cars, #4’s will work.(they’re really 4.5) I’ve got mostly #6’s and can run A Dash 8 and longer cars on them no problem. If your planning on running auto racks and long passenger trains, go with the #8’s. Larger turnouts are better if you have the room. (big steam needs big turnouts)

4

Mike,

The Atlas ‘Snap Switch’ has a continuous 18" radius curve through it. It is not really a standard ‘numbered’ turnout. It was designed for use with sectional trackage.

The Atlas ‘Customline’ turnouts have ‘numbers’ :

  • The #4 is really a #4.5 turnout. It divirges from the tangent at a 12.5 degree angle.
  • The #6 is a true #6 and divirges from the tangent at a 9.5 degree angle.
  • The #8 divirges from the tangent at a little over 7 degrees.

So the bottom line is that the larger the number, the more ‘gentle’ the turnout divirges from the tangent. That ‘number’ is the rate the dirivirging leg moves away from teh tangent. A #6 divirges 1 unit to the side for every 6 units forward. A #8 divirges 1 unit sideways for every 8 units forward.

Jim Bernier

5

The turnouts with a 1/3 section of 18" radius curve are called snap switches and are constructed with an 18" curve through the diverging leg which is why they have no turnout number. The 1/3 section of curved track makes it a drop in replacement for a full 18" curve section.

The turnout number refers to one unit of divergence per turnout # units of tangent distance. Thus for a #6 turnout the tracks are 1" apart for each 6" along the straight leg. For a #8 turnout they are 1" apart for each 8" along the straight. Obviously, you can curve the tracks beyond the frog so this ratio only holds for as far as both legs are straight. Thus a number 6 is sharper than 8.

The NMRA has a detailed rp for HO turnouts here http://www.nmra.org/standards/rp12_3.html but some manufacturers deviate from these dimensions. As noted above Atlas #4 is really #4 1/2 which makes it suitable for 22" curves.

Enjoy

Paul

6

IR:

From which the OP will note that a true #4 actually contains a sharper radius than a snap-switch. I remember obtaining a number of old Casadio no. 4’s at a train show and being extremely confused when they wouldn’t “work” with my Atlas no. 4’s.

(I doubt there would be any problems running equipment over such a short section of 15"R)

7

A good topic - I have wondered about the different numbered turnouts; this answered my questions.

8

The only problem I have heard of with true #4’s is that fixed long wheel base equipment such as bigger steam locomotives and possibly the 3 axle trucks in 6 axle diesels may have trouble with the 15". Probably why Atlas (and some others) increased their #4’s.

Enjoy

Paul

9

THINK OF ‘snap’ switches as ‘drop in’ for a 36" circle (18" radius). Simpler for beginners.

NMRA numbered (#4-#10 etc.) are straight through the frog (and beyond) like American RR’s use, to reduce derailments - frog picking - etc. They do not make a circle. (only the points are curved.

  1. Not all brands’ same numbered switches are the same. Some are tighter than others. Some offshore brands use metric (Kato, Peco, Tillig).

THINK OF #4s is for 4 wheel trucks and #6s for 6 wheel engines; or #4s for ‘yards’, #6s as ‘mainline’ and #8s as ‘high speed’ - at least for 98% of todays equipment.

REAL RR’s use #10’s for 5 mph, where Model turnouts are generally made to fit on plywood boards. The higher priced brands seem to be more accurate, look better, and have less derailling.

Simply, some longer equipment wants wider curves (Wathers full length passenger cars for example).

10

John Armstrong in “Track Planning for Realistic Operation” has a chart of the different turnout sizes, including their replacement radius and their closure (tightest) radius (as noted for example, a No.4 has a closure radius of 15" in HO). IIRC A no. 5 turnout has a closure radius of 26". It gets bigger as the number gets higher, I think a no. 8 is the equivalent of something like a 100" radius curve.