The Union Pacific Railroad's claim that these were the biggest locomotives in the world was not exactly true, but they were impressive pieces of machinery weighing in at just under 600 tons and capable of developing 6000 HP. They were built to haul heavy freight over the mountains in Wyoming and were sometimes used in tandem on the long, heavy grades. They were coal burners, and being stoker fired, produced prodigious amounts of smoke when working hard. This is what the gentleman who commissioned this painting remembered from his youth and although large amounts of smoke are signs of inefficient combustion - How can you argue with someone who has actually witnessed the passage of these monsters? Four cylinders and sixteen driving wheels must have been an unforgettable sight.