4Union Pacific Railroad Heritage covers the history of this amazing railroad that was founded in 1862 and completed the United States first transcontinental railroad in 1869. With the need to develop more powerful steam locomotives to handle the railroad's steep gradients, the Union Pacific Railroad designed the 4-12-2 locomotive, 4-6-6-4 Challenger which influenced development of the 4-8-8-4 Big Boy, followed by the 6,600-horsepower Centennial diesel locomotive, and 8,500-horsepower gas turbine electric locomotive. The Union Pacific Railroad operated well-maintained passenger trains including City of San Francisco, City of Los Angeles, City of Portland, and City of Denver until May 1, 1971, when AMTRAK took over United States intercity passenger service.