The "Big Boy" is considered to be the world's largest successful steam locomotive. The locomotive was used to haul the heavy freight trains over the mountains between Cheyenne WY and Ogden UT. The "Big Boy" is an "articulated" engine that is 132 feet 9 1/4 inches long. It weighs 600 tons and could generate a speed of up to 80 mph. The Union Pacific railroad ordered 25 units and, of that number, seven are on static display and one has been restored and is fully operational. Hear more here.