In the mid-1980s, demand for export coal necessitated the introduction of the C class bogie hoppers on Ngakawau-Lyttelton coal trains.
In the early 1990s, rail operator Tranz Rail began experimenting with the use of diesel traction through the Otira Tunnel, using modified DX class locomotives with low-level intakes and modified drawgear (locomotives modified in this manner were later reclassified DXC).