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Which were the biggest locomotives ever built? The world's biggest locomotives were 534-ton steam locomotives, nicknamed 'Big Boys'. Built for the Union Pacific Railroad in the early 1940s, they hauled 70-car freight trains, weighing 3,000 tons, at speeds of up to 80 mph (129 kph). 'Big Boy' locomotives were built to haul heavy freight trains over the Rocky Mountains.   Previous: Back Book: 1001 Section: Science and Technology Chapter: Railways