Should I use CAD to design my layout?
Layout design software is useful for
If you get good enough at a software package you may find you can doodle track plans faster on the computer than by hand. BUT, no layout design package out there will "design" your layout for you. They will not tell you about an imbalance in the number of staging tracks vs number and length of yard tracks vs car spots at industries.
They won't tell you that your planned operating scenario just plain won't map very well onto the trackage you've planned.
And for the most part trying to run trains on the computer to validate an ops plan is mostly a gimmick/waste of time.
The software packages are basically smart pencils.
If you'd design a lousy plan using pencil and armstrong squares you'll design a lousy plan with a cad package. If you'd design a brilliant plan manually, you'll design a brilliant plan with cad assist.
The computers are also good at giving precise information
about benchwork widths and where to locate the center of a curve.
That said I've found 3rd Planit to be **very** useful over the years. I've designed 2 layouts with it (plus some modules. I've designed a house and use it as a general purpose cad package for things like string diagrams for scheduling trains or 3d illustrations
I typically no longer try to design a railroad down to a gnat's eyelash. Instead I just use it to make sure that what I want to do with a space will fit (with a little slop).
I find that often, as I'm building the physical layout I have other ideas regarding laying out trackwork and I'll incorporate those changes as I build (sometimes going back and putting them into the 3rd Planit file).
I've used 3rd Planit to tell me the lengths of a series of joists projecting from the wall when the fascia has some curve to it and also to give railhead heights when curvy track is on a grade.
One final question for you: How much experience do you have with layout design, construction, and operation? If the answer is "not a whole lot" I'd **strongly** suggest NOT embarking on a "dream" layout. Because as your interests, skills and knowledge change you likely won't think of your basment monster as a "dream".
A 3d illustration generated with 3rd Planit
Detailed construction plans generated with 3rd Planit
A 3d layout mockup allowing deck visibility checking.
Which CAD program is best?
I use 3rd Planit. Cadrail is another good program. Some have used Auto-Cad (an expensive general purpose CAD program) for model railroad design.
I'd recommend staying away from Atlas's Right Track program.