[?]

How to:
1.  Find an object that is about the right size and shape for a piece of machinery.  I used a spare box from a railroad station kit and a random shaped piece from an old sign.  Just about anything will do.  Place them on a piece of glass.
2.  Next fill a small dish (I use an old Cool Whip dish) with Matte Medium.  Matte Medium is sold in art stores or under the Woodland Scenics name at most hobby stores.
3.  Take tissue paper, any color will do (I used green), and crumple it up a few times.  This will make it more flexible.  Then dip a 4 inch square sheet in the Matte Medium.
4.  When it is significantly soft and wet, pull it out and place over the objects on the glass you are using for the machinery.
5.  Let the tissue paper dry.  When it is dry, pull the object off the glass and cut the excess off, to leave a nice tarp shape.
6.  Paint the tarp blue or black.  Dry brush it with lighter shades of blue or black to give the tarp a nice used look.
7.  You are done.  Put the tarp on any flat car to represent a piece of machinery being moved.

Below is a pic of the underside of my two machinery pieces.  Notice the tissue paper was originally green and I painted it blue.  The load on the left is almost completely covered with paper.

Below the tops of the two loads.  They are ready to go on a flat car.

Below are the two flat car loads on one of my WM flats.  I would choose slightly larger loads when I do more of these, but for now these are fine.  If you want, you can "tie" the loads down with black thread for an even more prototypical look.