Sub-Classes
There are two sub-classes of the DX - DXB and DXC.
The classifications DXB and DXC reflect modifications that have been carried out to the fleet. These include heavier drawgear, uprated engines, new cabs, DC class-style low short hoods (some have received DFT-style low hoods) and higher power traction motors.
The designation DXB denotes that the locomotive has been fitted with the Brightstar electronic traction control system, while DXC denotes that the locomotive has been additionally fitted with modified air intakes (“chutes”) as well as all the other upgrades including Brightstar. Thirteen of the original batch of 15 have been upgraded to DXB standard (DXC 5039 and DX 5045/DXC 5520 being the exceptions), and most of those from the subsequent batches upgraded to DXC standard - exceptions being DX 2639 (long since withdrawn from service), DX 5448 (downgraded from DXH to DXB as a result of frame damage sustained at Waipahi in 1999) and those rebuilt to DXR (DX's 5362 and 5235).
Generally the DXC sub-class is assigned to the South Island, as the modified air intakes were designed to assist in are useful for working the Midland Line, coal trains (see above) particularly in the formerly electrified Otira Tunnel section, whereas the DXB sub-class were predominantly assigned to the North Island. Following the introduction of the first twenty DL class locomotives, four DXB's were reassigned to the South Island for general freight service on the Picton-Christchurch-Dunedin-Invercargill route, while DXC class locomotives have appeared in the North Island while on shakedown runs after overhaul.
The plain DX class is designated by GE as U26C, the newer upgrades are designated C26-MMI (2,750 hp DXB/DXC) and C30-MMI (3,250 hp DXB/DXC).
Read more about this topic: New Zealand DX Class Locomotive