To continue to meet increasing demand for coal haulage, more locomotives of the 4000 Class type were ordered. An initial order for 15 units was placed during 2006, with a further order for another 15 units placed during 2007.

Due to several design changes, these locomotives have been allocated as the 4100 class, to distinguish them from the 4000 class. Changes include a slightly different, Tier 2 emissions compliant engine, Mitsubishi traction motors rather than the 4000 Class’ Siemens traction motors, and a change from the ICE computer control system to the FIRE computer system.

In 2011, a number of 4100s have been moved to Western Australia and subsequently renumbered as the ACN class to assist with ore contracts.