Track Curve Radious Requirements

0-31 maybe but if you cannot do large curves 0-36. The larger the curves the more real estate needed for them. You have to consider the space you have for the layout and take into consideration if you are going to buy larger engines which might not take kindly to the tight curves and tight switches. To your choice of track that is entirely your choice but good information has been given on the availibilty of track and switches. I am a long user of Gar Graves track and Ross switches....................Paul 2

O72 has a [nominal] circle diameter of 72 inches and thus a radius of 36". Nominal because some track brands/styles are measured center-rail to center-rail (Lionel Fastrack) while others measure outside edge of ties to outside edge of ties. For me this is a case of bigger-is-better because to me trains look more realistic on broader curves but it depends on your interests and what you are trying to do. And yes, big locomotives require big curves, some a MINIMUM of O72. I use O72 curves with the exception of an O36 curve that I hid in a mountain so I didn't have to look at it. 

Most locomotives generally will let you know their minimum curve requirements, either on the box or in the instructions.  Which doesn't necessarily help if you purchase a used locomotive with no box or instructions.

You will need to find out the minimum curve requirements for the biggest locomotive you have and/or plan on purchasing in the future, and design your layout from there. 

In general the wider the radius/diameter of curves the more realistic it looks, but of course that is where compromise comes in (and I am going through this myself, my wife claims me trying to design a layout is worse than a woman shopping for a wedding dress....).  An O72 radius circle would take around 16 pieces of track (it depends on the brand), and would be anywhere from 72 inches across to maybe 73", depending on how they measure the radius. In general scale size equipment takes wider radius curves, the  big steam engines, 20+ inch passenger cars and scale size freight cars. Some engines will run on smaller size rail, but they may not look all that great. Traditional post war O equipment was designed to run on relatively small radius/diameter curves with some exceptions, so if you are running that you may not need 072 level.

It comes down to what you are planning to run. While anything looks better IMO in wider radius curves, if you are planning to run semi scale or smaller scale units, you may not need O72. It also depends on your space, on my layout my main loops of track are O72 and O64  at the moment, because  I have at least one scale sized engine (an older Williams brass Hudson) that pretty much needs the wider radius, it means though having less room for other features I would like to have and also meant, because I didn't want to have a table with removable access panels, that I don't have the ability to do a reversing loop. 

Even if you are running semi scale if you have the room I recommend going with diameter above O36 for main  loops and try to use as large as can fit and meet your needs. I used small diameter curves in some industrial areas/dock scene on my layout, because I love small units like docksiders and plymouth diesels working that kind of trackage, while my mains are larger radius for the through trains. 

There are wider curves in O27 than 27".  If you don't want to spend a ton of money, use your O27 straight tracks (lower height than O) and buy some 34", 42", 54" or even 72" diameter curves in the lower O27 profile. The 27 is a bit of a misnomer and not a limitation to tight 27" diameter curves.  K-Line and Lionel also made/make turnouts in 42".

You can make compound curves as well. For example some 042 sections with 054's leading and exiting gives you a center to center dimension of 44". The larger radius sections help ease the trains into the curves.

This is gargraves w/ wood ties