During the 20th century, Wall Street continued to grow and expand, and it played a central role in the development of the US economy. In the early 20th century, Wall Street played a key role in financing the growth of American industry, including the expansion of the railroads, the rise of the automobile industry, and the development of the electrical power industry. During World War II, Wall Street played a critical role in financing the war effort, and it continued to play a key role in the post-war economic boom that led to the rise of the middle class in America.