Since the opening of the original New York City Subway line in 1904, multiple official and planning agencies have proposed numerous extensions to the subway system. One of the more expansive proposals was the "IND Second System", part of a plan to construct new subway lines in addition to taking over existing subway lines and railroad rights-of-way. The most grandiose IND Second Subway plan, conceived in 1929, was to be part of the city-operated IND, and was to comprise almost of the current subway system. By 1939, with unification planned, all three systems were included within the plan, which was ultimately never carried out. Many different plans were proposed over the years of the subway's existence, but expansion of the subway system mostly stopped during World War II.