As the population of New York - and specifically Manhattan - grew throughout the 19th century, boundaries grew strained and surface transportation increasingly difficult. However, through the vision and financial backing of a handful of wealthy investors, by the early part of the 20th-century New Yorkers were living in the outlying boroughs of The Bronx, Queens and Brooklyn, and commuting across the rivers into Manhattan on a system of public transportation that in scope is still unrivalled.