TrackDraw is a program by Cary Clark to allow planning of train (and other type) layouts.
Trackdraw is implemented for Wintel machines and uses .net technology. It is open source, and uses an XML based internal structure for menus, parts libraries, and designs. There is an active community of folks modeling parts for it.
TrackDraw is not format compatible with Track Designer, an older program for designing, planning and documenting train layouts.