These are my personal opinions only -- your mileage may vary! I have
no commercial ties with any of these developers of model railroad layout design CAD except that I own and use 3rd PlanIt (and previously owned earlier revisions of CADRail). For the balance I looked at free demos.
found that 3rd PlanIt was a bit more consistent with my expectations of Windows software with an overlay of CAD human interface.
Some of the other programs have a user interface more in keeping with traditional CAD packages. Best to try before you buy.
For the Mac, one of the best-known
fully featured programs is RailModeller. I have not looked it in some time, but new versions have been released recently.
Some people use general CAD programs (such as AutoCAD)
for layout design and that may work reasonably well if one is familiar with them. The downside is that you will probably
need to create your own libraries of turnouts and other track components. Further, if you are not careful in the creation
of templates it can be easy to place commercial track components in unbuildable configurations, such as turnouts too close
to one another. Model railroad-specific layout design CAD software avoids many of these issues.
CADRail are probably the most popular for Windows, but nearly all the programs offer a free trial copy with varying degrees
of functionality. If you are serious about buying a CAD system, that's doubtless the best way to go ... but it will
take some time to evaluate multiple packages
At one time, I had a more-detailed listing of CAD suppliers on this page, but it had fallen far out-of-date. In fairness
to those developers, I have removed that section and will update it if I have time in the future.
Any of the model railroad CAD programs have a substantial learning curve and
for a one-time job you might be better off with paper, pencil, and templates. I personally believe that paper and pencil are better for many designers than CAD, especially in the early design stages when creativity is more important than precision.
A final thought when considering
any program ...
Many of these packages are written, sold and supported by very small companies or individuals. The risk of some
of these small developers going out of business is real. Good advice is to save versions of your trackplan to standard file
formats such as .jpg, .dxf, and .dwg. And back-up your files to CD or other media often. (You do make back-ups, right?)
Another risk to consider is that developers may change file formats and in the process obsolete track plan files
developed in earlier versions. Even if you are happy with the program you have chosen, it pays to check in with their users
groups or official website occasionally so you'll be aware of any such changes.