In the late 19th century, as America's teeming cities grew increasingly congested, the time had come to replace the nostalgic horse-drawn trolleys with a faster, cleaner, safer, and more efficient form of transportation. Ultimately, it was Boston, a city of so many firsts that overcame a litany of engineering challenges, the greed-driven interests of businessmen, and the great fears of its citizenry to construct America's first subway.