The Union Pacific pioneered trancontinental rail service. As the railroad grew, 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 the Union Pacific 'Big Boy' 4-8-8-4, and Challenger 4-6-6-4, to engines working local and branch lines, like the Pacific 4-6-2 and the Consolidation 2-8-0.