The part about about needing to dream it yourself is absolutely right. What I have found CAD most useful for is checking elevatons. Bob T
For me, I use 3rdPlanit. Its very good software for general-purpose track planning. I use it much in the same way that you might use a pocket calculator or a spreadsheet program in some other context. Once you get the hang of it, its very handy that way.
3rdPlanit is extremely flexible and is a true model railroad CAD program. I think its learning curve is kinda steep, but that said, once you put the time investment in to learn it, the results are spectacular and well worth your effort, IMO.
I originally set out to "design my layout" with 3rdPlanit, which was the beginning of a very interesting journey. Partly from the "learning curve" of the software standpoint, and partly from the learning curve of designing layouts in-general standpoint.
I must have designed and re-designed and designed again over and over my layout a bajillion times. And in truth at times it seemed more an exercise in discovery of what I *didn't* want or wouldn't work more often than the reverse. But I stuck with it and eventually the software didn't seem so unfamiliar or unwieldy anymore and I had an excellent concept of what would and could fit into my space.
One of the very first things I did-- and I *STRONGLY* recommend the same to you, whatever software you use or whatever your approach to your design is-- was to make an *exacting* plan of the basement space. I measured everything as carefully as I could, noted all the "problem" areas and/or spaces I could / couldn't use, etc. Also note that these items may exist at different *heights* as well. You might, for instance, only have a problem with a particular location at one elevation but not another. Those are things you should note on your basement space plan.
And then of course I did that again because the first time I didn't think of everything or measure carefully enough. And then again, and again-- whenever I discovered a discrepancy between actual measurements and the space plan in the computer. No matter what else you do, having a good, solid, accurate space plan to work from is the very best gift you can give yourself. It is very surprising how easily mistakes, oversights, or omissions can creep into your space plan-- especially if you have a "challenging" space to work with like I do. Heck, even a year later I found a major discrepancy involving whole feet of space that required me to get the tape measure back out and make some very basic and fundamental changes to my understanding of the space. I hadn't noticed it sooner because I hadn't been doing much with that part of the room-- and then came along to actually build something there and discovered some original measurement errors-- which can happen easily if you're working with a space that's bigger than your tape measure.
In time, and through many repetitions, it became clear-- to me and with respect to my space-- that the railroad could pretty much only go one of a few ways, and that 80-90% of the benchwork would be the same regardless of whatever track plan I ultimately chose. That might be the case for you, it might not. I think many designers would suggest you design the track plan and then fit it into the space. In other words, design the benchwork to the railroad and not the other way around. But whatever works, IMO. So eventually I just said "to heck with it" and started building benchwork to fit the spaces I knew had to be benchwork. I also know essentially where all the mainline will go, so I've got that covered too. Now its just a matter of Figuring out where I want to place all my towns and industries and the rest of the trackplan, I think, will come together.
I have gotten to the point now of using 3rdPlanit like a "pocket calculator" of a sort for designing the layout. Whenever I have a question about what can work or "what would the grade be" or "can i fit that into this space" or whatever-- I pull out 3rdPlanit and my space plan and quickly "mock-up" whatever it is I'm wondering about and can very quickly determine the answer and move on. And 3rdPlanit allows me to check all the dimensions, and to go view it from any angle or even "fly around" it in 3D mode-- which I have to admit is pretty much fun :-)
I don't know if any of this is how other people design their railroads, but for me, the CAD program is really more of a tool I use to help figure out and document what I'm doing, not the other way around. Which is not the understanding or approach I had when I started. Originally my intent was to "sit there until I figured it all out" and then go build it. Whether that's a workable approach may also depend on how much space you have available to model in, and/or how complicated it is to work in that space. Or perhaps how easily your "vision" comes to you and you're able to picture your railroad being completed. For me though, designing the railroad and working on building it has been more of a process and a journey than a beginning and an end. I wouldn't have figured that either when I started. Other people's mileage may vary-- I can only speak for myself in that regard.
What other people have been saying to you-- about "Givens and Druthers" is absolutely correct. I heartily recommend that one of your first real "planning" steps be to sit down and think hard about what you really want, what your resources are, what your space looks like, etc-- and put it all down on paper-- organized or not, just get it down-- so you can start really seeing and honing and refining your goals. Maybe you'll be able to "hit it in one" and maybe it will take a number of rounds worth of refinement. But either way, you'll be glad you went through the exercise and when you're done you'll have two really good and solid things to guide you in your journey-- your space plan, and your wants, desires and limitations.
Good luck to you!
Modeling the South Pennsylvania Railroad ("The Hilltop Route") in the late 50's