XTrackCAD is a CAD program for designing Model Railroad layouts.
XTrackCAD supports any scale, has libraries of popular brands of x
turnouts and sectional track (plus you add your own easily), can
automatically use spiral transition curves when joining track
and has extensive on-line help and demonstrations.  
XTrackCad lets you manipulate track much like you would with actual
flex-track to modify, extend and join tracks and turnouts.
Additional features include tunnels, 'post-it' notes, on-screen
ruler, parts list, 99 drawing layers, undo/redo commands,
benchwork, 'Print to BitMap', elevations, train simulation and
car inventory.