Overcrowding had been a problem at Park Street since the Main Line was routed through it, and did not improve when it reverted to streetcar-only operations. The problem worsened with the influx of passengers transferring at Park Street Under, as the only entrances to those platforms were narrow staircases connecting through the streetcar level. In preparation for the Boylston Street subway and the extension of the Cambridge subway, the Boston Transit Commission substantially expanded the streetcar level. The platforms were extended to the south and straightened; the southbound platform was increased in area by one-half, and the northbound platform by three-eighths. The south headhouse of the southbound platform was moved south to the end of the expanded platform. The construction work began on August 7, 1914, and was completed on March 8, 1915.