Atlas Switches

Picture of a 7.5 switch top.

#5 switch bottom.  Note that the 7.5, longer, has less diverge angle.

O-xx turnouts have an actual curve in the diverging route of the turnout, whereas numbered turnouts (i.e., #5, #7.5 in Atlas-O products or #4, #6, #8 or #10 in Ross products) allow for much more gradual diverging routes in the turnout -- the higher the turnout number, the more gradual the diverging route.  #5 indicates the diverging route measures 1 diverging (vertical) unit for every 5 linear (horizontal) units, whereas a #7.5 indicates the diverging route measures 1 diverging unit for every 7.5 linear units, etc... (see Mike's photos in the above post for a visual representation).

Numbered turnouts are DEFINITELY the way to go when connecting parallel mainline tracks, since they avoid the S-curve inherent with O-xx turnouts.  Atlas-O's track geometry typically provides for 4.5" center-to-center parallel track spacing.  And both the #5 and #7.5 turnouts yield this spacing when used as mainline cross-overs with the diverging routes connected together.  Theoretically speaking, #7.5 turnouts would provide greater "high-speed" transition of trains between mainlines vs. #5 turnouts -- especially for trains consisting of longer, scale-length cars.  But practically speaking, #5's will work just fine as mainline cross-overs unless you really have the real estate for #7.5 turnouts.  #7.5's will definitely need more linear space to accomplish the same function as #5's, but the sleek look of trains traversing #7.5 cross-overs is really cool.  Gotta see it to appreciate why those high-numbered turnouts are built the way they are. 

If you have the space, I'd stick with numbered turnouts throughout your layout.  And only use the O-xx style turnouts for a tight branch-line or industrial spur where you don't have the necessary real-estate to accommodate numbered switches.  O-xx turnouts are typically NOT appropriate for yards, since they'll create track spacing that's unnecessarily far apart.  Best to use #'d turnouts in a yard, where center-to-center track spacing can drop to less than 3" as needed to increase yard capacity.

BTW, watch out for those 89' boxcars on O-63 curves.  That's a pretty sharp curve for scale-length freight and passenger cars.  Proceed with caution... and stick with O-72 minimums for your mainline curves -- and even at that, those 89' cars will have noticeable overhang inside the curve.  And you may find the 4.5" center-to-center spacing with Atlas-O's standard sectional track pieces may not provide sufficient clearance when operating longer scale equipment on lesser-radii parallel curve tracks.

Hope that helps!

David

Thanks for the advice Rocky Mountaineer.  Would 5" spacing be sufficient then for the larger scale cars?

Atlas sectional curves will produce a 4.5" spacing.  The diameters differ by 9". 

The a crossover using the Atlas #5 turnout or the #7 will maintain that 4.5" separation.  If you wanted a 5" (and additional 0.5 inches) spacing you would needed to insert 2 x 1.25" sections or 3 x 1.25" sections of track respectively.

Here are some informative data sheets from the Atlas O website which you should find useful. 

Jan

True.  However, a cool technique to yield a bit more center-to-center spacing on the curves -- which I first saw on Norm C's layout -- is to stagger the start of the curves a bit.  You can play around with RR-Track software or whatever track planning software you care to use to get the desired effect(s).  But it works great, 'cause you can maintain the 4.5" centers on the straight-aways which is more than sufficient.  Yet on the curves, you'll wind up with a slightly variable center-to-center distance throughout various portions of the curve, but it will always be greater than 4.5".  The maximum distance will occur at the 90-degree portion of the arc and will be based on how much you offset/stagger the start of the curve. 

When done properly, you don't notice the curves aren't 100% concentric anymore, but it's a subtle enough difference that avoids any side-swiping issues on parallel curve tracks when running articulated locomotives, 89' freight cars and full-scale 21" passenger cars.  The sharper the radii, the more you may need to offset entries to the parallel curves.  If running Atlas-O O-99 and O-108, you may not need to do this at all. Whereas if you're running O-72 and O-81 curves, the offset curves could save a bunch of headaches down the road when running the longer equipment.

Best of luck... and have fun planning!

David