In 1902, WAGR ordered twelve Ten-Wheelers from Baldwin for the Murchison line with a low permissible axle load. They were a scaled down version of the Ec class Pacifics that had also been delivered by Baldwin a year earlier. They got the nickname “Baby Yanks” because the Ec were also called “Bull Yanks”. The letter C was used to designate the class, which had previously been assigned to a class of tank locomotives with a 0-6-0T wheel arrangement that had been retired in the same year.

In contrast to the Ec, the class C received only a simple two-cylinder engine, which further reduced the production costs. From 1908 they received a trailing axle, which led to the 4-6-2 wheel arrangement and the renaming to class Ca. Another series of ten locos was built directly in the Midland Railway Workshops in 1915 as 4-6-2. With the installation of a superheater, they became the class Cs from 1929. The WAGR used the last locomotives until 1961. Some locomotives had been used in industry since the early 1950s, and these were also scrapped in 1964.

In the 1920s, WAGR found that it did not have a sufficient number of suitable locomotives for long-distance express trains and that the 1902 E-class Pacifics were no longer up to date. The main aim was to eliminate the need for helper engines on the Darling Scarp's inclines and to reduce the number of stopovers to replenish supplies with the help of a more economical engine. It was hoped that the journey time on the Trans-Australian Railway's own section would be significantly reduced. In order to use the low-energy coal from the Collie fields, the new locomotive also had to have a large grate, which was easy to implement with the design as a Pacific. 

Ernest A. Evans developed a Pacific, which was influenced by the New Zealand Railways Class A^B and the Tasmanian Government Railways Class R, and had a Belpaire firebox. The first ten examples were made by North British in Scotland in 1924 and 1925. In 1927 and 1929, a total of 15 more from the WAGR's own Midland Railway workshops followed. They initially received new tenders, but some were later converted to shorter tenders from the old R class.

Since towards the end of the thirties there were still many old locomotives in stock that had to be replaced with more modern ones, ten more class P locomotives were ordered. They received various improvements in details in the areas of boiler, bogie, headlights and valve gear. As the first WAGR locomotives ever, they were given names, which in this case came from Western Australian rivers. Thus they were initially only known as the River class, but in 1946 they were given the new designation Pr.

With the boiler pressure increased from 160 to 175 psi, these enignes were able to move trains trains of 555 long tons at gradients of 1 in 80. The axle load could be switched between the original 12.8 long tons of the class P to 14.2 long tons. Initially, the new locos were used uniformly with the old locos, with the boiler pressure set at 160 psi while not making use of the higher axle load. Only later were the safety valves adjusted differently and the device to change over the axle load used. At the same time, eight pieces of the original design were brought up to the same level.

During the Second World War, the P and Pr classes were often used for troop transport. After the war they were temporarily converted to oil firing when there were problems obtaining coal from the Collie fields. They were supposed to be replaced by the Pm and Pmr classes, but this did not work due to their poor running characteristics and the P and Pr classes remained in express train service. The boilers were later brought up to the same standard as the Pm and Pmr. After the introduction of the X-class diesel locomotives in 1954, the steam locomotives were relocated to freight and shunting service. They were finally retired between 1967 and 1969 and today one P and one Pr each survive.

The class W was created after the Second World War as a Mountain with a low axle load, which was mainly intended to pull freight trains and, if necessary, passenger trains. The original design of the WAGR was fundamentally adapted and standardized by Beyer, Peacock & Co. before production started. The first 40 locos were only pre-assembled in England and completed in Australia, while the last 20 were delivered ready to run.

The running gear was basically based on that of the South Australian Railway class 400 Garratt, which was built as a “Double Mountain”. The boiler was built with a combustion chamber, thermic syphons and arch tubes and was optimized for the inferior Collie Fields coal.

Despite the standardization, the locomotives had a few teething problems, which initially led to many breakdowns and required changes to the locomotives. The Silverton Tramway received four identical locomotives, which were also designated as class W. The WAGR retired them between 1968 and 1972. Today, 15 of the 60 locomotives are still ain existence, some of which are regularly used on tourist trains and others are being overhauled.

Since the Western Australian Government Railways didn't have enough operational locomotives after World War II, they ordered a modernized variant of the pre-war class P that was to be designated class Pm. Ten of 35 locomotives were to be built at their own Midland Railway Workshops. But due to insufficient capacities, all 35 were delivered by North British and put into service in 1950 after they had been shipped to Australia.

As with the P and Pr, 19 Pm were built with plain bearings on the driving axles and roller bearings on the carrying axles, while the 16 Pmr had roller bearings on all axles. One Pm was later rebuilt to a Pmr at the Midland Railway Workshops. Due to their rough running characteristics, they were quickly relegated to express freights and used on the Great Southern and South Western lines. Withdrawals came between 1970 and 1972.