Unfortunately for the sake of an easy answer, there isn’t one. As you can see, everyone approaches this from a learned way. Each of us has to find a system that works, refine it when it seems right to do, and then move on.
I negotiate the space first, and then assign the maximum mainline running at scale speed that I can generate with at least some changes in elevation to give the overall appearance of the layout some gee-whiz factor. So, I doodle. I’ll take an entire evening while my wife watches her seemingly endless cloning of CSI, and draw different boundaried shapes for a layout. Then, when I have a few of those, say three workable spaces that offer decent access, I’ll go downstairs, scan them, and make five or six copies of each.
As the evening wears on, I draw and erase, adding this, leaving that, and come up with at least two very interesting or appealing possibilities. These all have to have the five-eight absolutes, what we call givens. For me, they would be fast mains, tunnels, at least one timber trestle, a yard, decent passing sidings, an engine servicing facility, and a reversing loop for turning entire trains. Chip would ask me what about a double-slip switch. Yeah, there’d have to be one of them in the yard. That sort of thing.
Once I managed to get them on a plan, I would start to tinker to find which plans offered changes in elevation that I could fit in the space without making the whole thing look heavily contrived or toylike…Disney-like. Actually calculating, using arithmetic, what grades would be required for a given height and the distance I can use to rise to that height, I modify what I must in the curvature and crossing points where one track crosses over another.
I find that simply doodling helps to achieve the order that satisfies. Sometimes the impetus comes from someone else’s work, sometimes I want to start and finish it all myself. Few of us will be original…it