The DX class was introduced in response to a requirement for a more powerful locomotive to handle traffic on the North Island Main Trunk (NIMT).

[1] Before their introduction the heaviest freight and passenger trains on the line required two members of the 1,060-kilowatt (1,420 hp) DA class to haul them.

[8] Their introduction led to a dispute between New Zealand Railways and the Engine Drivers, Firemen and Cleaners' Association (then the trade union representing NZR locomotive staff) over additional pay.

The union argued more powerful locomotives meant less work for its members and successfully took NZR to the Government Railways Industrial Tribunal for additional pay in July 1972.

[9] Initially, the class were confined to the North Island Main Trunk as their 16.25-tonne axle load was too heavy for many of the bridges on the other lines.

[10] The second phase DX had a number of improvements including additional dynamic braking, larger sandboxes and a better excitation control system.

From their introduction in 1972, a member of the class was assigned to haul the prestigious Silver Star overnight NIMT passenger express.

In 1988, a major reshuffle of the mainline locomotive fleet took place nationwide as a result of the opening of the NIMT electrification and introduction of the EF class to handle the central section.

For the DX class, this meant leaving the upper North Island and being split into two fleets for service between Wellington, Gisborne and New Plymouth, and one in Dunedin.

The class now operate in the South Island, predominantly hauling coal trains between the West Coast mines and the port of Lyttelton near Christchurch.

The class is also involved in general freight operations and is employed periodically to haul the TranzAlpine and Coastal Pacific passenger trains.

A small number, including both DXRs, were based in the upper North Island, primarily in general freight use on the ECMT.

The class were also periodically used to haul through trains on the central NIMT when EF locomotives are unavailable or the electrification requires maintenance.

The DX class was originally numbered from 2600 to 2648 – which in common with NZR practice of the time was based on power output (2600 hp).

"[19] The bogies and fuel tank were painted a flat grey colour and North American automatic couplers were fitted – these being soon changed to a beefed-up version of the NZR chopper-type.

Between 2013 and 2016, the DXC class were fitted with fire suppression to run the TranzAlpine through the 8.5-kilometre (5.3 mi) Otira Tunnel on the Midland Line due to a change in requirements resulting from the Commission of Inquiry in the Pike River Mine disaster, and as well on any other passenger trains on the Main North and Main South lines.

These include heavier drawgear, uprated engines, new cabs, (some have received DFT-style low hoods) and higher power traction motors.

DX5362 had been stored out of service at Hutt Workshops since early in 1989 suffering a cracked frame, and therefore was an ideal candidate for the rebuilding programme.

[20] One locomotive was however rebuilt to these original DXR specifications during the Tranz Rail years, DX5310 in 2001 for its return to service following its derailment at Pukehou while hauling the Bay Express in 1995.

Following the purchase of Tranz Rail by Toll in 2003, the new management authorised the rebuild of DX5235 – itself having been stored since being damaged in a collision in Christchurch in 2000 – which became DXR 8022.

The new DXR featured minor differences in the carbody hood and a MkII version of the "Universal Cab" which was boxier than the previous MkI variant as applied to DXR8007.

[23] The locomotives are to be overhauled with 3,300 hp-rated GE 7FDL-12 EFI diesel engine prime movers with electronic fuel injection.

[23] Also involved in the overhauls will be the strengthening of the underframe, and fitting of new 2MN maintenance-free draw gear, electronic braking system and full locomotive rewire and new engine oil coolers.

The locomotives will be reclassified as DXE,[24] will have a newly designed long-hood (similar to the DXRs) fitted with chutes, and the short-hood will be lowered.

[4] In August 2017, KiwiRail announced they were in discussions with global suppliers including General Electric, Electro-Motive Diesel, Alstom, CRRC and Stadler Rail to construct replacements for the remainder of the class.