I would recommend adding "car float" as a search term. That's what the flat deck barges with tracks on top pushed by tow boats usually were called.
A couple of nice pictures of a Walthers car float being assembled and a prototype photo of a CNJ (Central of New Jersey) car float with a
small diesel switcher and a couple of cars on:
http://www.bronx-terminal.com/?p=1608. Walthers is reissuing their car float, apron and tow boat in feb 2009.
But I think Ralph's solution - start with a piece of woodwith a couple of tracks on top, nicely painted, detailed and weathered is the best solution for most of us. That is one good looking car float, Ralph!
Smile,
Stein