I suppose you'll be using Atlas Custom line turnouts,wich I don't know much about so I won't comment.However,I know no.4's are very tight so you can forget running a Big Boy through them.As an example,Athearn specify a minimum 12 in. radius for their Big Boy in curves wich translate to "medium" turnouts in Peco,wich I know better.And even then,a turnout is a special beast to deal with,so I'll go with "large" turnouts where I intend to run my Big Boy.
If you wish to run most anything,you'll have to settle for rather large curves and fair radius turnouts if you don't want your layout to be plagued with constant derailments.Big steamers and passenger trains with long cars need large radiuses (12+ in.) to prevent derailments and even wider curves (15-16+ in.) to look good.The wider the better.....
You may be able to stay with no 6 turnouts in yards since speeds are much lower,but still,while most of your rolling stock will be happy with it,your Big Boy and Challenger may not.
As listed by the NMRA, center to center distances for a Class I railroad on a tangent center to center is 1 1/32" and for a 14.5" curve is 1 1/4".
I wish I could give you a definitive answer on the switches, but I can't.
I suspect that the #4's you are talking about are the Atlas standard switches, while the #6's are the listed #6's. I have never checked the #6's, but I have looked at the Atlas Standard Switches.
The Atlas standard switches do not seem to be straight #4's as defined by the NMRA when measured. They have dimensions that match both a 4 and a 5. I measured an equivalent radius of a little over 14" for them with roughly a 15 degree divergence angle. So they have a divergence of a four with a curve radius roughly that of a 5. Seeing they are longer than a NMRA standard #4 the divergence angle could be achieved with a larger curve radius.
Of course, the proof is in the pudding. On my present layout, I have a mix of the standard switches and the #6's. I use the number sixes for cosmetic purposes (trains just look much better going through the six than the standard) as my minimum track radius is 13". I run a Rivarossi Big Boy and the new Athearn Challenger and have no problem negotiating either switch type. I have yet to purchse and operate the Athearn Big Boy. I also run Kato's California Zephyr with no problems with either switch.
The only steam locomotive I have had problems with (even on the 13" radius) was the Concor War Baby GS. No problem on the switches of either type, but it didn't like the 13" curves until I "tinkered" with it.
I am presently designing a new layout using principally the Standard switches, as they have an euivalent radius close to the design "minimum" of 16" radius curves. When I'm done, I'll go back and replace sidings and other mainline turnouts with 6's where I can.
My recommendation is use #6" where you can and the Standards where you can't. I don't think that you would have any problems with the Standards in your yard or engine facilities, I havn't.
Jack W
You'll be happier if you stay with #6's.
I am code 80, n-scale as well and I have a 4-8-4 steam that just gives me random troubles over the #4's I've got. Also, in my yard, I initally placed a few turnouts too close and had to move them because the 4-8-4 overhang was enough to "bump" the remote switch on the turnout (where the selinoid(sp) is for the turnout).
If you're not using remote turnouts it's probably less and issue but the #6's just look better IMO.
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