The literal, physical construction of the American railroads was accomplished by a diverse and often exploited labor force, with the Transcontinental Railroad being the most famous example. The Central Pacific Railroad, building from the West, relied almost exclusively on Chinese immigrants—over 12,000 men—who performed the most dangerous work, including blasting through the granite of the Sierra Nevada mountains with nitroglycerin. Meanwhile, the Union Pacific, building from the East, was primarily powered by Irish immigrants, many of whom were veterans of the Civil War. In the American South, the railroads were largely built through the labor of enslaved African Americans and, after the war, through "convict leasing" programs that effectively continued the practice of forced labor. These workers lived in harsh "Hell on Wheels" mobile camps, facing extreme weather, disease, and hazardous working conditions for very low pay. Their physical toil—laying every spike and rail by hand across thousands of miles—was the "brute force" that physically unified the North American continent and laid the foundation for the modern industrial economy, often at a tragic cost of human life.