The sections from Plimmerton to South Junction, north of Pukerua Bay and Muri, and North Junction to Paekakariki were duplicated in 1940. From 24 July 1940 electrification at 1500 V DC of the southern section of the NIMT from Wellington to Paekakariki was completed. The Tawa Flat deviation has a long tunnel (Tawa No 2) not suitable for steam operation because of excessive smoke (although steam trains were temporarily operated in the new deviation from 1935). A Centralised Train Control (CTC) system was installed in 1940, so that new signal boxes were not required and five stations between Tawa and Pukerua Bay no longer had to be continually staffed for Tablet operation; see Kapiti Line. Electrification eliminated the need to relieve the steep (1 in 57) gradients from Plimmerton to the Pukerua Bay summit by a deviation to the east and allowed more frequent suburban passenger trains (and allowed suburban electric multiple units to run on this section from September 1949).