The Union Pacific pioneered transcontinental rail service. Eventually, running freight through half of America meant long consists hauled by some of the largest locomotives ever built. Reaching back to the end of the steam era, Union Pacific pictures giants ranging from mainline Union Pacific 4-8-8-4 'Big Boys' and Challenger 4-6-6-4s, to the most powerful single-unit locomotive ever built, the 6,600 hp EMD Centennial. Steam on! 13.75 x 20.6 inches, open.