The construction of the Sixth Avenue line was very difficult because of the various utilities and tunnels above, below, and beside the line. At the time, it was considered the costliest subway line in the city. Builders had to use very small charges of dynamite so that they would not disrupt the H&M tunnels alongside the route, the street and elevated line above it, and the water main below it. The Sixth Avenue Elevated had to be underpinned during construction, adding another $4 to $5 million to construction costs. The Catskill Aqueduct was located around below the avenue's surface, and workers on the new subway had to be careful to not cause any cracks in the aqueduct. As part of the construction of the IND line, the H&M's 14th Street and 23rd Street stations had to be rebuilt to provide space for the IND's 14th Street and 23rd Street stations, which would be located at a similar elevation. The 19th Street station was not affected because the IND tracks were located below the H&M tracks at that point. However, the 33rd Street station had to be relocated to the south of its existing location, above the new IND line. The IND platforms were to be located at the same elevation as the present H&M station, and there was no room to build a new subway station either above or below the level of the existing H&M station.