The DX class was initially used for heavy freight trains on the NIMT, especially on the central section where the power was needed. Most of the 49 class members in service by 1976 were used on this line, but a few also operated between Palmerston North and New Plymouth via the Marton - New Plymouth Line most days of the week, and also occasionally on passenger trains between Wellington and Masterton on the Wairarapa Line. In 1974 a DX travelled on the North Auckland Line as far north as Whangarei, and thereafter they saw occasional use in Northland in the late 1970s and early 1980s but did not have any regular assignments there.