Like all other electric locomotives in New Zealand, the leading letter of the locomotive's classification is E. There are two predominant theories about how the ED class acquired the second letter, D. The first is that it comes from the "Do" of its 1-Do-2 wheel arrangement. The second is from its original allocation to two locations, Wellington and Otira - Arthur's Pass, hence "duplicated". Official records provide no confirmation of either theory.
Introduction
New Zealand Railways purchased one ED class locomotive in 1938 from English Electric, No. 101, for use on the newly opened Tawa Flat deviation, which incorporated two long tunnels. This locomotive was known as "The Sergeant" because of the three longitudinal stripes on each side of the body that were unique to this locomotive. The tender required the supply of locomotive components for the other locomotives required, as it was thought desirable to carry out manufacture in New Zealand in NZR workshops. A further seven locomotives were assembled at the Hutt Workshops, and two at Addington Workshops for use on the Otira - Arthur's Pass section of the Midland Line. The two South Island locomotives were later transferred north.
Renumbering
With the introduction of the Traffic Monitoring System in 1979, the two remaining locomotives were renumbered ED15 and ED21.
Steam boilers
Each locomotive originally had oil-fired water-tube boilers for passenger carriage steam heaters, supplied by the Sentinel Waggon Works. The boiler could supply of steam per hour at a pressure of, and the water and oil tanks had capacities of respectively, so could steam for four hours before refilling. However they were shut down or removed in 1950 due to "on-going reliability problems"; air turbulence particularly in tunnels or when trains passed on double track resulted in downdraughts affecting the boiler and in passenger discomfort in winter. In June 1951 the DME said that the cost of fitting suitable boilers for the section from Paekakariki to Wellington was not warranted as the carriages leaving Paekakariki had residual heat, and a steam loco could pre-heat carriages before they left Wellington. In 1954-55 two boilers were installed in the Wellington station basement. The CME then wanted eight locos to have boilers for the 1955 winter, but parts were not available for the obsolete boilers and "refurbishing did not proceed". It was also found that the boilers were unreliable as the burners had been amended to be outside the normal operating specifications.
Withdrawal
With the introduction of DA class diesels on the Paekakariki via Pukerua Bay to Wellington electrified section in 1967, eight of the class were withdrawn from service in 1969 and scrapped. The remaining two were kept in sporadic service until March 1981, when both locomotives were sold into preservation. There were plans to send them back to the Otira - Arthurs Pass section but nothing came of this. ED 101 is preserved by the Silver Stream Railway, while ED 103 is preserved by the Canterbury Railway Society.
