Track Planning Software for MAC

http://www.railmodeller.com/

What Mike said

Without installing emulation software Railmodeler is your only option. best part it's a pretty good program. 

I am curious who will make a rail design app first

This has been discussed before.  There is also Empire Express, but it hasn't been updated in a while, and XTrackCAD which is free.  I've used the free EE in the past, and really liked it.  I'd buy it and use the full version if he ever updates it.  XTrackCAD is harder to use, but the price is right.  On some systems it can be buggy.  Also drop the SCARM creator a line letting him know you are another Mac user that would like his software ported over.  He has said if there were enough interest he'd look into doing it.  But as a Mac user, I have found the best thing is to use SCARM while booted into Windows.  But then I have my Mac set up to use Windows for other reasons (Games and software for bringing work home.) when I need it.

Jim, you need to turn off the setting in System Preferences> Security > General tab. "Allow PP"  to run the installer. Once installed you can turn it back on

If you get that message, you might wish to change your settings in the 'Security' system preferences. 

If you change the 'Allow apps downloaded from:' setting to be 'Anywhere', then the app should download and run.

Then you can change the setting back to a more secure option afterwards if you wish. 

XtrackCAD will also need X11 installed to run, unless they have a newer build (I haven't used it in a long while.)