I have both Macintosh and Windows boxes available to me...
I tried 3rd Planit back in 1999 and found it too complex. I reopened it today and confirmed.
I found Empire Express for the Mac. Fairly easy to use, but I don't like the look of the track. Some useful libraries, but not much structure support.
I found RailModeler for the Mac. Superb interface and extremely easy to use. But lacking in libraries.
Took quick look at XtraCAD. Looks easier than 3rd Planit, and the price is right. Tentatively looks like they have a lot of nice libraries, but the online docs are lacking... so until you download it and get up to your waist in it, you don't know what it has!!! Would be nice if they'd get a page up somewhere listing all the included libraries. I appreciate that this is open source, but compare it to JMRI, which has an excellent array of pre-use documentation online.
My next step is to download XtraCAD and give it a shot.