I was thinking of stealing...er...borrowing your container textures to use in making a cargo car. Yes, the scale would be off in the length, but I think it could work.

Gimp or Photoshop and you can stretch that puppy out to make it work!

Really the tough part about that texture was pulling the original from the UM:BoM dumpster. (took awhile to find exaclt which piece I wanted to use.)

Shoot, I lost my paper modling links. A lot of modern locomotives are boxy, so they should not be that hard to make a simple representation.
Also there is quite a lot of use for one or more cars left on a side track, especially to serve business. While there is a main swtiching yard to make and take apart strings of cars for moving between cities, many business that actully make use of shipments by train can be miles from that swithing yard. Mayhem industrial might make a good werehouse for a frieght office. So yes I can see uses for cars without a loco.