New Guy,
You won't need a computer application. I sure didn't. (And, for 80 years, nobody building O gauge layouts ever felt the need for one.)
Only use 031 curved pieces, and 10 inch straight pieces, and just fit them together like you want, just like LEGOS. It is extremely easy. The worst that happens is that in hooking the major sections together, you find that a 10 inch straight pieces is too long to join the sections together, in which case you just cut the straight piece shorter to make the fit.
Or, if the end of a completed half curve (i.e., 4 curved pieces hooked together creating the end of a loop) does not come down far enough to hook into a particular straight run of track (i.e., the end of the curve needs to extend down further to make the connection) then you just cut a short piece of straight track, and insert it into the center of the half curve (i.e., between the first section of two curved pieces comprising the first a quarter of the turn, and the second section of two curved pieces comprising the second quarter of the turn), to bring the end of the curve down longer (vertically) so it hooks into your long section of track. Just measure how far you need to bring the end of the turn down to match up, and cut a short piece of straight section to that length, and put it into the mid point of the half curve.
Often, because of the mathematics of the ordinary 10 inch straight and O31 curved sections, the shorter straight pieces you will need are just the standard "half-length" sections, which are 5.5 inches and readily for sale. (I discovered this by accident after hand cutting some short pieces to make the fit.) So, I would recommend buying at least four of the half sections when you build your layout. Believe me, you will need them.
I will be glad to send you the picture of my layout, and when you see it, you will know that every curve on the end of a loop is just 4 pieces of O31 curved track, hooked together, but often will a short piece cut to be inserted in the center of the loop), and every straight run of track is just several standard 10" straights hooked together, with an occasional short straight piece in it to make things fit.
I think that in total, I only have about 6 pieces of short-cut straights in my entire layout. Pretty easy to do, but you must have a good steel ruler with precise markings, and not a flexible tape measure or child's one foot ruler, which is often "off" by an eighth of any inch. And, sometimes you just "blow" the cut, and have to recut a section.
You must also absolutely have a set of O-Gauge track adjustment pliers, which are pretty cheap. That are precisely made to match the tops of the steel rails so that you can re-crimp the ends of the track when you put the pins in and hook the track sections together. Try using any other type of plier, and you will damage the ends of the track, really for no reason. You can find these plyers for sale on Ebay, Amazon, and also the sponser companies of these boards.
As others will tell you, for any given layout configuration, the length of your short cuts will vary, because your short sections must be cut so that the length and width of the layout configuration will fit on your board. (And in some cases, you will need to omit an entire 10" straight piece, to get things to fit on your board.)
My layout is a variation of the simple classic "figure 8 inside of a large oval" layout. Take a look on google, for images of train layouts, and you will probably see several of these. I am sure that other members here can post a diagram of this classic configuration as well. By using 4 switches in the layout, you will be able to have one train running inside the oval, on the figure 8 section, and another train running on the outside oval, at the same time, without running into each other. Or, you can run one train, and have it travel the outside oval, and then turn inside to run on the figure 8 section, and then out again to the outside oval, in one long travel route, by just properly setting the four switches for the trip and leaving them alone.
Another advantage of this classic layout, is that it leaves two large circles inside of your layout, to set up little scenes.
I know that all of this may seem complicated at first, but it is really very simple.
Mannyrock
