The current 63rd Street lines were the final version of proposals for a northern midtown tunnel from the IND Queens Boulevard Line to the Second and Sixth Avenue Lines, which date back to the IND Second System of the 1920s and 1930s. The Second System was a plan to expand the city-owned and -operated Independent Subway System (IND), which often ran in direct competition with the two privately owned subway companies in the city, Interborough Rapid Transit (IRT) and Brooklyn-Manhattan Transit Corporation (BMT). As a result, the plan for the line only had it connect to two planned IND lines, the Second and Sixth Avenue Lines.