New Zealand DX class locomotive

The classification of the new locomotives was announced as the "D^X" class.[7] Diesel mainline locomotive classes begin with "D", followed by another letter, however "X" was out of sequence. Up to the D^X class introduction, all classes had used letters A to J, and shunting locomotives used S.

The first 15 members of the class (D^X 2600 - D^X 2614) arrived in Auckland in February 1972.[8]

Their introduction led to a dispute between New Zealand Railways and the Enginedrivers, 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. The additional pay was known as a "horsepower allowance".[8]

The locomotives were progressively introduced into service from November 1972.[9]

Initially, the class were confined to the NIMT as their 16.25-tonne axle load was too heavy for many of the bridges on the other lines. As it was, several bridges and viaducts on the NIMT had to be strengthened to take the weight of the locomotives. Their prime movers were found to be less reliable than NZR was accustomed to with its other locomotives.[9]

The first DX class locomotives proved to be a success.[10] With bridges progressively strengthened throughout the North Island, and to further increase capacity on the NIMT in particular, 34 more members of the class were introduced in 1975 and 1976.[10] The second phase DX had a number of improvements including additional dynamic braking, larger sandboxes and a better excitation control system.[10][9]

D^X 2612 (DX5137) at Avondale in Auckland in the late 1970s. Note the two small windows in the cab of the locomotive, replaced in 1988 with a single-window as a result of single-crewing.

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.[10] 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.

From their introduction in 1972, a member of the class was assigned to haul the prestigious Silver Star overnight NIMT passenger express.[10] Previously, the train required two DA class locomotives. The DX class was also used on the Northerner overnight passenger train when it was introduced in 1975.

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.[11] This was the first use of the class on the Palmerston North - Gisborne Line and the northern section of the Wairarapa Line between Masterton and Woodville.[12] At the same time, experiments began with DX class locomotives in the then electrified Otira Tunnel.[13] While the initial tests were unsuccessful, later testing in 1991 found that modifying the locomotives with special air intake ducts and adding ventilation fans would allow the DX class to be used without electric locomotives.[13]

In the mid-1990s, a small number of DXs returned to Auckland for use between Auckland and Tauranga via the East Coast Main Trunk, on the Kinleith Branch and regular service between Auckland and Whangarei. This was the first use of the class in regular service east of Hamilton. In 2000, a DX ventured north of Whangarei, which remains the last expansion phase of DX class territory.

Two DXC locomotives on a coal train from Ngakawau to Lyttelton crossing the Waimakariri River bridge

The DX class is still used primarily on heavy freight trains. The majority of 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 trains consist of up to 30 CE and CB class hopper wagons each carrying up to 52 tonnes of coal, and require five members of the class to haul the train up the 3% (1 in 33) west-east grade of the Otira Tunnel,[14] and two members to haul it on the other sections of the line. These services are now exclusively hauled by the DXC sub-class. The class are also involved in general freight operations, and also employed periodically to haul the TranzAlpine and Coastal Pacific passenger trains.

In the North Island, the DXB sub-class were the primary haulers of milk trains to Fonterra's Whareroa factory near Hāwera on the MNPL, usually operated in pairs to haul these trains up the 1 in 35 Westmere Bank north of Wanganui. The class is also involved in other freight operations in the lower North Island, usually operating singly or in multiple with other locomotives.

A small number, including both DXRs, were based in the upper North Island, primarily in general freight use on the ECMT. With the allocation of the new DL class locomotives to that area, these locomotives were transferred to the South Island in late 2011.

The class were also periodically used to haul through trains on the central NIMT when EF locomotives are unavailable or the electrification requires maintenance. They were the primary haulers of the Capital Connection service between Palmerston North and Wellington.

In October 2013, KiwiRail reassigned six DXBs to the South Island due to the commissioning of the second batch of the DL class, leaving three assigned in the North Island.[15] All of these locomotives, including the units assigned to the South Island prior October 2013, were transferred back to the North Island to cover for the DL locomotives in March 2014, as the DL class were temporarily withdrawn from service after they tested positive for asbestos.[16] All units are now operating in the South Island as of March 2020.

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).

With the introduction of the computerised Traffic Monitoring System (TMS) in 1979, the class was renumbered. The DX class received four-digit numbers starting with 5, in which the fourth number is a check digit for the number.[17] The locomotives were numbered in order, with D^X 2600 renumbered DX5016 (6 being the check digit) and D^X 2648 renumbered DX5517. The numbers 500x, 515x, 534x and 549x were not used as these numbers do not generate a valid check digit, and D^X 2639 never received a TMS number as it had been withdrawn by then.

The DXB and DXC sub-classes have kept their original TMS numbers, while the DXR class received new TMS numbers beginning with 8, numbered irrespective of their DX number. DX5045 received a new TMS number using the DXB algorithm (5520) when it was upgraded and returned to service following being involved in a fatal derailment at Te Wera in 2002 (when re-introduced into service, the DXB classification was used to denote what is now the DXC sub-class).

DX 5356 leaving Avondale in the late 1980s. The locomotive is painted in the International Orange or "Fruit Salad" livery of NZR from the late 1970s onwards. Note the single front window above the short hood.

DX 5097 in Cato Blue livery with Tranz Rail logo, in Auckland, 2001

DXC 5258 in Toll Rail Corn-Cob livery at Otira in July 2008

Upon introduction, all the DX class members were painted in a livery consisting of the locomotive body painted orange except for a chrome yellow segment that curved down from the top of the locomotive's long hood and ran back to its trailing end. The cheat line separating the orange from the yellow was approximately half-way up the side of the locomotive. The reasons for this livery have not been established.[18] This livery was known as "Clockwork Orange" after the Stanley Kubrick film "A Clockwork Orange."[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 1977 and 1986, the entire class was repainted in the International Orange "Fruit Salad" livery - red, grey, and yellow, which most of the class wore until the end of the decade. This livery had the road numbers displayed on the sides of the locomotives in large white numbers with DX 2612 the first to be painted as such. Since then, members of the DX class have received liveries including Cato Blue (Fruit Salad with blue instead of red), Bumble Bee (black and yellow), Corn-Cob (green and yellow) and the KiwiRail phase 1 and 2 schemes (grey, orange or red, and yellow). No DXs remain in the Fruit Salad, Cato Blue, Bumble Bee and Corn Cob liveries.

The locomotives received an upgrade during 1988 with the introduction of single-manning to improve visibility for the driver, with a large single front window replacing the original two small windows.

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. Because of this, the number of Otira bankers have gone from three to five, which allows any locomotive without the system to be shut down. In mid 2016, the number of bankers were reduced from five to four. As of December 2016, all 32 units have been fitted with fire suppression.

DXR8022/DX5114 on a train from the Racecourse Hill Siding at Darfield are held at Heathcote for DX5264/DF7132 on a local shunt and the return W4 coal empties, 2021

DXR 8022 and DXB 5143 at Ethelton with a container train from Picton to Christchurch

In 1993, New Zealand Rail Limited undertook a significant rebuilding of DX5362 into the first of what was to be a new sub-class, the DXR. 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. Work began to fit the locomotive with a new 3,300 horsepower (2,500 kW) GE 7-FDL diesel engine, upgraded traction motors with a ratio of 5.17 to the then-standard DX ratio of 4.55, and a new design of "Universal Cab" with a shorter low hood and more angular appearance. The use of 'R' (like the DBR class) denotes that the new locomotive was a rebuilt DX.

The newly rebuilt DX received the number DXR8007 when it entered service in 1993 along with the prototype DFT class rebuild, DFT7008. Initially, it was intended that New Zealand Rail would rebuild all of the DX class locomotives to DXR specifications similar to the programme of the rebuilding of the DF class into the DFT class, but after the privatisation of New Zealand Rail Limited (renamed Tranz Rail in 1995) later in 1993, the new management decided not to undertake further DX class rebuilds, due to the cost.[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.[21] It did not though receive a new DXR style cab, so it retained its original DX classification until it was overhauled to DXC standards in 2010.

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. The new DXR rebuild was classified as DXR8022 (due to DXR801 not generating a valid check digit) and entered service in late 2005. It was decided to rebuild DXR8007 to match 8022 in the interests of standardisation, and so 8007 was again rebuilt at Hutt Workshops in 2006.

The two DXRs on the Waipara River Bridge with a freight train in November 2016

Upon each rebuild, GE classified 8022 (and reclassified 8007) as model C30-8Mi. As rebuilt by New Zealand Rail, DXR8007 was classified by GE as their model C30-7M.

In 2007, DXR8022 was one of the locomotives involved in a biofuel trial using a fuel mix of 5 per cent biodiesel and 95 per cent regular diesel over a period of six months.[22] With the introduction of the DL class locomotives into service the two DXRs were transferred to the South Island, and are now used between Middleton and Picton.

At 2,420 kilowatts (3,250 hp), the DXR class was the most powerful class of diesel-electric locomotive ever operated in New Zealand, until the introduction of the DL class in 2010, which surpassed the DXRs power output with 2,700 kilowatts (3,600 hp).

In mid-2018, it was announced 15 DX class locomotives are to be overhauled, and given an electronic upgrade.[23] The locomotives are to be overhauled with 3,300 hp-rated GE 7FDL-12 EFI diesel engine prime movers with electronic fuel injection. They will also be given the latest version of the Brightstar engine control system, new engine control and wiring harness, and new high capacity fuel tank, along with a new battery box assembly.[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.[23] It is planned this will give the locomotives a life extension of 20 years.[23] As of early 2020, the project is on hold due to the cost of rebuilding coming close to, or exceeding the cost of buying new locomotives.[4] 5500 is currently stored as an under-frame.[4]

DXC 5212 is the first of five locomotives to be fitted with the new EFI prime movers, but remains as a DXC due to the cancelling of the DXE project.[25] As of May 2021, 5212 and 5454 have overhauled and have been fitted with the EFI prime mover. 5391 is currently under overhaul, and will be the third to receive one.