You can buy ready-made loads, or as noted above, make your own.
These lumber loads are made from scale stripwood (basswood), and are removeable. To save material, the piles are hollow.
This pole load is made from dowels from the lumber yard, tapered a bit with a rasp or some coarse sandpaper, and coloured with woodstain from the hardware store.
You can also build industrial-looking loads from styrene: these crane parts make up a load for a complete train.
You can also use Bic pens (the round, white ones with the coloured caps work best). The outer barrels make a good load of heavy pipe for a gondola or flatcar, and the ink refills, with the writing ends cut off, make good small-diameter pipe loads. Paint to suit, and add some stakes and stickers as required. Sorry, no pic of these available.
Wayne