Since I make my living, such as it is, using and programming AutoCAD, that is what I've been using for my track planning. The drawback is the ease with which I can sit down & redesign the entire layout. Again, and again and again. Started planning the new layout May 9th of this year and finally got around to the "foam-breaking ceremony" last week on the 2'-6 x 7'-6 sawmill module for the Muddy Creek RR. I designed my last house in less time. (And I expect, the layout will take longer to build than that house did.)
Since this layout will be a series of modules or dioramas located around my living & work space & connected by long, super-lightweight removable sections, I can still spend countless more months at the computer endlessly redesigning the balance of the layout while making some actual progress. Even managed to run a train up and down the mountain, something I haven't been able to do in AutoCAD.
Wayne