To allow NY&H trains to continue running during the project's construction, the tracks between 120th Street and the Harlem River were moved to the east side of the river. On January 5, 1873, the tracks between 116th Street and 120th Street were moved from the center of Fourth Avenue to the east side. In December 1872, shafts were sunk at 91st and 93rd Streets and two tunnels were being built alongside the old tunnel. The tunnels were to be completed in September 1873. A temporary trestle bridge was constructed from 97th Street to 120th Street for $85,000. In May 1873, temporary tracks were placed on the west side of Fourth Avenue; trains went over Lenox Hill instead of through the tunnel.