What major infrastructure was developed in the United States with the help of Chinese immigrants?

The development of railroads in the United States, particularly the Transcontinental Railroad, was significantly aided by Chinese immigrants who arrived in the mid-19th century. After the Gold Rush, many Chinese workers found employment opportunities in railroad construction due to a labor shortage. They played a crucial role, especially in the Rocky Mountains and the Sierra Nevada, where they undertook some of the most challenging work, such as laying tracks and blasting tunnels. Their contribution was vital in completing the railroad that connected the East and West Coasts, facilitating trade, travel, and the overall expansion of the nation.

This historical context emphasizes the vital role that Chinese immigrants played in shaping the American transportation infrastructure during that period. Highways, airports, and subways were developed later and were not primarily associated with the labor of Chinese immigrants in the same way that railroads were. The significant and transformative impact of railroads on American society during that era underlines why this option stands out as the correct answer.