They had to deal with rock formations and ground water, which required pumps.
Sections of the tunnel, as well as the Cortlandt Street station, which was directly underneath the Twin Towers, were severely damaged.
The most grandiose IND Second Subway plan, conceived in 1929, was to be part of the city-operated IND, and was to comprise almost Though most of the routes proposed over the decades have never seen construction, discussion remains strong to develop some of these lines, to alleviate existing subway capacity constraints and overcrowding, the most notable being the proposals for the Second Avenue Subway.
Plans for new lines date back to the early 1910s, and expansion plans have been proposed during many years of the system's existence.
The city built most of the lines and leased them to the companies.
these now operate as one division called the B Division.
Since the IRT tunnels, sharper curves, and stations are too small and therefore can not accommodate B Division cars, the IRT remains its own division, the A Division.