Coal is transported by train in massive freight cars known as hopper cars or gondola cars. At the mines, conveyor belts or chutes load the coal into these cars. Hopper cars have sloping bottoms with trapdoors, so the coal unloads quickly using gravity. Gondola cars, on the other hand, need rotary dumpers that flip the entire car upside down. Often, long “unit trains” with more than 100 cars move thousands of tons of coal in one trip—hauling it from mines to power plants, steel mills, and shippin