For decades gold was the main medium of exchange for banking and commerce in a goodly part of North America and especially the U. S.  western territories and states.  The transfer of gold in bullion or in raw dust and nuggets, while not routine, was a regular occurrence, especially in the vicinity of mining operations.  Hence the temptation by certain individuals with a nefarious bent to earn their "living" by helping themselves to the cargo in special stages or fitted wagons carrying gold.  Gold is heavy stuff.  A half-ton of solid gold would be a little less than a cubic foot were it ever minted in such great blocks.  It takes up more space as dust, nuggets or bars but for practical purposes is easier for handling and, luckily for robbers, easier for dividing when carrying it away. These shipments were heavily guarded as you would expect.  Still, where there is a will, there will be a way figured out.  Dangerous work for everyone involved.