Go to Great Models (see earlier ref) , go to Trumpeter, click on their new 1/35th scale locomotive (this sucker will be 35" long and 5" high), click on "accessories," and voila! you'll see a bunch of 1/35th scale German RR cars, including flat cars, armored trains, etc. under the company name "Ironside."
I personally use "G" scale stuff and find it works well - far better, for my taste, than "O" scale, which I find much too small. The G scale flat cars will hold two Shermans comfortably. Bachman used to offer kits of various kinds of rail cars you could assemble yourself that were really good value. Even much of their preassembled rolling stock is reasonably priced, especially in relation to companies like Aristo, LGB, Pola, Lionel, etc. They also used to sell inexpensive plastic track in straight and curved sections. You might check with them to see what's still available.
Dick