Radii question, double mainline

I'm pretty sure that O36 and O42 will be close if not too close.  Same with O31 and O36.  The real test is what engines and cars will be running here, small stuff like a GP9 or a Consolidation steamer might skate by, anything much larger or cars longer than maybe 12-13" passenger cars are likely to be a problem.

Track spacing looks way too close for me, here is Menard's O31, O36, and O42 circles.  The distance between the rails for each circle is only 1.5", I'd never recommend anyone do that!

It depends.   The O36/O42 main line will have ~ 3" between track centers.  If you run postwar on the inner loop pulled by short locos (think Beeps), you might get away with it.  It's the conflicts of overhangs on the inner loop against the underhangs on the outer loop that you want to avoid and that is equipment-specific.  You will have to test with your gear.

Some things to try.  1.  don't make the circles concentric.  Start the O36 curve before the first O42 curve so you are not mixing overhangs and underhangs as trains pass.  2. Substitute an O31 piece or two for O36 to reduce the diameter of the the inner loop.  3.  Tubular track flexes (you may have to loosen the tabs on the ties to do this) so you can slightly enlarge the diameter of the outer loop.  You can also squeeze the inner loop (If you flex track, you may have to cut a short section of curve to make up for the change in circumference).

Are you wedded to a 4x8 platform using a single piece of ply or foam?  Try one of the free demo track programs (Anyrail, Scarm)  and come up with a plan you like.  Build the platform accordingly.

Here's what I believe is what you can do, but it requires cutting some 10" straight tracks for the Blue and Green ovals.

The Blue is O-42, Yellow is O-36, Green is O-31 and Orange also O-31. You can see how the Green can be used in place of the Yellow, but the curves will be a little tighter and overall not as concentric. If you want more separation and still be concentric, the Orange can be used in place of the Yellow and Green. I know the Green is a bit hard to see, but You should be able to see how the ends are pinched in a bit and the curves can be seen outside the Yellow. The center-to-center clearance of the Blue and Yellow is 4.5". If you need more, then you'd need to use the Blue and Orange with small filler pieces added to adjust the size of the oval as desired.

There's no way to use all 3 sizes because the clearance between the Yellow and Gold is only 3".

My double track mainline 072 & 063: spacing on the curve is 5.5" on center and 4.5" on the straight sections.   No problem with 18' passenger cars using the 063 loop meeting an MTH premier 2-8-8-4 Yellowstone running on the O72 0utter loop.  

Cheers, Dave

@DCtransit - I have a 4x8 layout with 042" inch curves on the outer oval and 031" curves on the inner oval and basically have done what @Tracker John has described above (particularly starting the inner curve before the first outer curve) and for the most part it works - subject to GRJ's comments about using smaller engines and cars on the inner oval.  In my case I prefer to run the trains in opposite directions so just to be sure, when I first put engines & cars on both tracks, if I have any reservations that it might not work (and sideswiping may occur) I'll start running both loops as slow as I can for a few laps to ensure that everything passes ok and ready to immediately kill power on both tracks if a collision is imminent.  

Hope this info is of some assistance to you.

In the past I have used short pieces of straight track to stagger the beginnings of the curves, but I also had a width wider than 48 inches to work with.  To check the spacing I would hand roll the largest engines and rolling stock past each other and adjust the tracks as necessary to get a comfortable clearance.  This was with mixed pieces of 031, 042, 054 and 072 tubular track.  The trains were postwar 15 inch pass cars, FMs, the 6518 depressed flat type cars, and a 773.

@DCtransit posted:

I purchased cases of Menards o31, o36, and o42.

What pair of radii should I go with ? Can I get away with o36 and o42 or will engines hit eachother.

4x8 bench work. double mainline

4' by 8' is just too small for a double mainline IMO. But it is your railroad and you can change it if you don't like it.