How about a regular CAD program...and someone who knows a thing or two about track design...surely your best bet. Not heard of any real track design software!
There are 2 free 2D CAD programs around at the moment...google the following:
DoubleCAD XT
Draftsight
Both written by respectable companies...not used either as I have access to AutoCAD, but have heard good enough things about them...and for free...who can complain!
Best bet for me though would be to design a space with features in it that you can lay multiple tracks on. Much more interesting than having the same layout each time...and much better for improving your driving generally...all the best tracks do this
