In 1927, automatic colour-light signalling was installed from Otahuhu to Mercer. In 1930 the signalling was extended to Frankton and the from there to Horotiu was doubled. The north to Ngāruawāhia was doubled from 5 December 1937, followed by Ngāruawāhia to Huntly on 4 December 1938 and Huntly to Ohinewai and Papakura to Paerata in December 1939. By then, wartime shortages delayed further double-tracking. Pokeno to Mercer was doubled from 11 November 1951, Pukekohe to Pokeno 21 November 1954, Mercer to Amokura 1 July 1956 and Ohinewai to Te Kauwhata 14 December 1958. The between Amokura and Te Kauwhata remain single track, as does Ngāruawāhia bridge.