In the 19th century, New York City was a burgeoning industrial and commercial metropolis - the largest city in the United States and second largest in the world. As the city's population increased, people began to call for construction of an underground railway. Many unusual engineering challenges had to be overcome, not the least of which was construction in a dense urban area. After lengthy legal battles over property rights and the debt limit of the city, ground was broken on March 24, 1900.
By the 1890s, the transportation infrastructure of downtown Boston - a maze of narrow, winding streets laid out, in some cases, along Colonial cow paths - proved completely inadequate for the needs of a modern, bustling metropolis. Tremont Street, the city's main thoroughfare, was regularly subject to gridlock from a convergence of foot traffic, horse-drawn conveyances, trolley lines, and electric streetcars. To rectify the problem, the Boston Transit Commission, with Howard A. Carson as chief engineer, was created in 1894 to study remedies.