In addition to threading around the H&M tunnel, the line had to pass over the BMT Canarsie Line along 14th Street, over the tunnels leading to Penn Station, under the four-track BMT Broadway Line at Herald Square, over the IRT Flushing Line at 41st Street, and under the 42nd Street Shuttle. Even though the line had to pass around multiple transit lines, the grades were kept to a minimum. Bellmouth tunnels north of 47th–50th Streets were built to allow for a future extension under Central Park and along Morningside Avenue to 145th Street. This extension was part of the Board of Transportation's long-range program, and was estimated to cost $34.914 million as of August 1940. Construction was expected to start some time after 1946.