The Union Pacific, incorporate in 1862, is the longest-operating Class I railroad in North America. Its rich history includes its role as the first U.S. transcontinental railroad, and that line became a key link for passenger as well as freight traffic moving from the Midwest and East to the West. The UP has also long been known for eclectic roster of motive power, which has included some of the largest and most-powerful steam and diesel locomotives ever built. The three-cylinder 4-12-2 and 4-8-8-4 Big Boy were iconic to the UP, as were high-horsepower GE gas-turbine locomotives; these were followed by big twin-engine diesels including the 6,600 hp DDA40X Centennial. In Union Pacific and Its Predecessors, Brian Solomon looks at the UP's history, including freight and passenger operations and motive power. He provides backgrounds of the many railroads that became part of UP, tracing how Chicago & North Western, Denver & Rio Grande Western, Missouri-Kansas-Texas, Missouri Pacific, Western Pacific, Southern Pacific, and others became key elements of today's Union Pacific system.