In November 1929, the New York Central prepared to install bidirectional signaling and shorten signal blocks between Grand Central and 162nd Street on the Harlem Division and High Bridge on the Hudson Division, allowing all four tracks in the Park Avenue Tunnel to be used in either direction. At the time, only Track 4, the westernmost track, has bidirectional signaling; it was used by outbound trains in the evening rush hour. The change would increase capacity by 25% and allow trains to operate at least 90 seconds apart at a speed of . The additional capacity was needed to accommodate growth in suburban communities and additional traffic to new skyscrapers near 42nd Street. Semaphore signals composed of pivoted arms were replaced with tricolor signals resembling traffic lights. The interlocking tower at 106th Street was rebuilt, ten new signal bridges were put up between 96th Street and the Harlem River. In addition, the line was rebuilt at Mott Haven Junction with nine tracks instead of the existing five, and four signal towers in the area were replaced with one. The changes at the junction were made to increase operational flexibility and allow trains to go through this section. In 1930, work on the section of the project between Grand Central and Mott Haven Junction was nearly completed.