The NZR class DG diesel locomotive was designed in the early fifties by English Electric as a response to a change in the dieselisation strategy. An order for 21 more 1,500-hp class DF locomotives was transformed into 42 of the new 750-hp locomotive. Now the standard procedure was to usually use the DG double-headed, while still having the flexibility to haul two lighter trains at the same time. In consequence, the DG only had one cab to save costs. The wheel arrangement was A1A-A1A to keep the axle loading at eleven and a half tons.
The 42 locomotives were built in 1955 and 1956 by Robert Stephenson & Hawthorns. Eleven of these, destined for use on the South Island, had received a device that could decrease the load on the unpowered axles, increasing the adhesive weight and the starting tractive effort. These could now reach 29,000 instead of 26,000 pounds and got their own class designation DH.
When more powerful diesel locomotives had been introduced and the DG and DH were only used for light duties, the load switching devices were removed from all DH in 1968. As reliability decreased in the seventies, ten were thoroughly modernized between 1978 and 1980. In this process, they also got a new cab, but in the result their reliability did not increase that much. So it only took until 1983 for all to be withdrawn. Four have been preserved, with three currently being operational.
