The ARG Q/4000 Class (originally Westrail Q Class) were introduced in 1997 to eventually replace the L/3100 Class. However they have ended up complementing them in service. The Q class were the 1st class in Australia to use the EMD 16-710G3B-ES engine producing a powerful 3800kW/4150HP.

Recently with the QRNational takeover, a new numbering scheme has been applied to the class to reflect horsepower (4000 Class).

The Q class are predominantly used west of Kalgoorlie, and rarely venture across the Nullabour.