I discovered this (ancient) thread during a search while planning a new model train layout and took a moment to update in case someone else (like myself) stumbles across it at some point in the future. I haven't tried either yet, so this isn't a recommendation.
Two OS X track planning software packages are:
Rail Modeller (
http://www.railmodeller.com/ )
shareware, USD $49 to register
version 4.0 current in January 2010
Empire Express (
http://www.haddonsoftware.com/index.html )
shareware, USD $34.95 to register
version 2.0.1 current in January 2010
Cheers