War trains dominated combat for more than 100 years. Massive railborne artillery shelled the enemy while trains unloaded troops and supplies. For a brief moment, the terrifying machines were the most powerful weapon on the battlefield. But technology advanced. Improvements to tanks, cars and planes during World War II marked the twilight of the war train. The great trains of the First World War still dominated the imagination, however, and the Nazis built impressive — but impractical — railborne cannons. The German Heavy Gustav was the largest gun ever built. It was more than 150 feet long, 40 feet tall and weighed almost 1,500 tons. The steel giant Krupp A.G. made only two, and neither worked well.