Dallas Model Works:But then that's the problem with Mr. Jobs little opus -- none but he makes software for it!
I don't know where you have been for the last many years, but there are hundreds of thousands of programs for the Mac written by thousands of software companies including Microsoft........ macs are not software challenged! Of course there are millions written for Windoze but the ones for the Mac just tend to work better (IMHO). Maybe you should do some research and have some facts at your disposal before you answer people.........
For the OP. Go to Apple.com and do a search for MRR Cad programs. There are quite a few MRR programs that are written in Open Source coding that work on all platforms. There is some shareware out there but I don't have any experience with it. I am using a program called Empire Express which is a download from Hadden Software: http://www.haddonsoftware.com/ Doesn't have a lot of whistles and bells, but gets the job done quite well. It has several different companies turnouts and track listed along with a lot of buildings for checking trackside clearance. etc. It will even print you out a shopping list based on your layout plan. i used it for the diagrams needed for the NMRA Electrical Certificate as well as for planning the new room for my layout. While it is not free, it is reasonable.
Good luck!