In the fifties, the WAGR placed the largest single Australian order for diesel locomotives to date with the class X. 48 were built by Beyer, Peacock and by Metropolitan-Vickers. With a 2-D-2 wheel arrangement, they were suited to be used on all lines due to their axle loading of only ten tons. Since many of the secondary lines did not have turntables, they got two cabs.

Their four powered axles were mounted in the rigid main frame. Power came from a two-stroke Crossley HST-Vee8 with exhaust pulse pressure charging. Although originally designed for 1,200 hp, they had to be de-rated to 1,045 hp due to several problems. Nevertheless, it still took a lot of work before the engines were running reliably. Due to the distinctive sound of these engines, the locomotives got the nickname “Hummingbirds”.

Originally, they were used for the most important express trains. 16 had been built for multiple working and with communication doors at each end for use with heavier trains, which were designated class XA. Starting in 1963, ten more received multiple controls, but did not get the communication doors of the class XA. To distinguish them, they were designated class Xb.

Over time, the class XA had to hand over express trains to more modern locomotives. In 1981, the ageing locomotives had to be down-rated to 875 hp. In their last years, they were mainly used for grain trains and transfer services. The first withdrawals had already taken place in 1973 and the last ones were gone by 1988. One standard class X and five class XA locomotives were preserved.