Here's a link http://www.nmra.org/standards/sandrp/rp12_3.html to a page with turnout dimensions. Keep in mind that manufacturers can deviate from these. As mentioned above, the Atlas custom line 4 is really a 4.5. The radius dimensions that are number 6 and number 11. 11 is the one of interest here because it affects long wheel base locomotives such as your 2-10-4. As you can see in the chart, it is 15" for a number 4, 26" for a number 5, 43" for a number 6, and 67" for a number 8. So a #6 or #8 should be just fine.
Here's a page with curvature/turnout recommendations http://www.nmra.org/standards/sandrp/rp-11.html Keep in mind these are minimums.
Finally here's the Layout Design SIG's page on curvature http://www.macrodyn.com/ldsig/wiki/index.php?title=Curve_radius_rule-of-thumb
From all of this you can see that ideally you should be using 36" curves and #7 turnouts. 30" and #6's will probably work okay since most equipment is manufactured to work on this or less.
As always, a little experimentation up front can avoid problems latter on. I suggest you buy a couple of turnouts and some flex track and try out crossovers and curves to see how well they will work.
Enjoy
Paul
You are right, loathar, I do know that the Custom Line version are the better of the two @ 4.5, and that the snap version is the curved radius version.
Thanks for correcting me.
-Crandell
#6 minimum. #8 preferable. Don't even think about #4's.
Selector-The Atlas Custom Line #4's are the 4.5 turnouts. The snap switches are the ones with the 18" curve diverging route.
Quick and dirty way to see what your rolling stock (not generic "2-10-4 and six-axle diesels") can handle:
Lay out 2 parallel track lines 2 inches apart. Draw crossovers with all the frog numbers you want to try, from #4 (8 inches along the track, plus closure rail curvature) to whatever (whatever X 2 plus closure rail curvature, which will be much longer than a #4.) Then take a piece of flex track (or 2, for the longer samples) and shape the curve. Power the flex with test leads and run the pride of the line through the simulated crossover. That will give you an answer that fits your roster, hard facts rather than guesswork.
I use this method to lay out the 'footprint' of specialwork that I'm going to hand-lay. There's no rule that says you can't do the same to help choose commercial products.
Chuck (Modeling Central Japan in September, 1964)
Grizlump, the Peco Streamline #6 turnouts are of the N. American configuration to suit our market, and are very good. I can get my BLI Pennsy J1 through the #6's with ease, as I can my Walthers heavyweights.
if you are going to run that 2-10-4 at all then you better use #6 as a minimum. #8 would be better and allow for higher speed operation. save the number 4's for industrial trackage where the big engines never go. i am assuming 2 inch track centers. if the c/c is greater then the reverse curve problem is not so bad.
i use the shinohara number 6's for crossovers and have no problems with 10 drivered engines but the peco number 6's can give you a fit since their diverting route is curved and you wind up with a severe s curve in the crossover. i am speaking about code 100 in HO scale. I have not used the later code 83 peco's.
grizlump
You might get away with a true #4, but I hafta tell ya...I wouldn't try it. Or, rather, I would...actually...I'd get my largest engine and car handy and see if they would negotiate a true #4 before I spent the dough for a bunch of them.
At the very least I would use a snap switch "#4" which is actually a #4.5 and would be considerably more trouble-free for you from the point of view of thinning your flanges or spreading your rails...or simply derailing. Even then, if you want room to grow over time, hoping to scratch an itch with a new large engine announced by someone, you would be much further ahead to use a simple #5. That turnout should handle almost anything except brass engines. As it happens, I only use Peco Streamline #6 Code 83 insulfrog turnouts as my smallest frog number, and the rest are curved and straight 7.5's and 8's.
In HO scale, code 83 shinohara track. My largest engines would be a 2-10-4 or 4-6-6-4 articulated and 6 axle diesels (PA's), largest freight will be 86' hi-cube box cars with coupler pockets that swing. Minimum radius I'm planning is 30" mainline and 24" siding / industrial areas.
Question is in planning the layout, what would be the absolute minimum turnout size for mainline to mainline crossings for the larger engines and keeping in mind space, that oh so limited space. I think #4 would be ok using the 4 axle switchers or a 4-6-2 steam.
Thanks, RF&Prr