As a result ten new P class locomotives featuring detail improvements to boilers, valves and bogies were introduced in 1938, a year before the outbreak of World War II.

[2] The Annual Report of 1920 pointed out the large numbers of obsolete locomotives in service and steadily growing rail traffic, stressing the need for more powerful engines.

[2] In 1923, approval was given for the construction of ten new superheated Pacific type locomotives for operation on heavier mainline rails, suitable for the hauling of the expresses.

[1] The P and Pr class also featured innovations to alter the weight-distribution between the driving and trailing wheels, improving adhesive traction.

[1][8] The next batch of ten P class locomotives were constructed locally in Western Australia in 1927, at the Midland Railway Workshops.

They were identical to the North British locomotives excepting the short tenders rebuilt from those of the obsolete R class engines.

By the late 1930s, with even more engines requiring urgent maintenance and repairs, orders were placed for an additional ten P class locomotives which would be improved through modifications to the boilers, bogies, headlights and valve gear.

[7] The boilers featured an increase in pressure from 160 to 175psi, which provided more power, and the bogies were constructed in cast steel.

[5] These improved locomotives were delivered to the same operating specifications as the original P class, with the same weight distribution and boiler pressure limited to 160 psi, though improvements to the track and bridges on the Eastern Goldfields Railway in 1940 meant that they could be altered to use their designed power[5] through the increasing of the axle load from 12.8 to 14.2 tons[2] and the resetting of the boiler-top safety valves to 175 psi.

[4] Following the war ten of the class were converted to oil burners in 1947 and again in 1949 due to industrial trouble on the Collie coalfields, where the WAGR obtained its coal fuel.

Experiments into engine pooling with the P class on the Great Southern Railway in 1932 led to the adoption of this practice across the system, freeing up further locomotives for other duties.

[5] The introduction of the Pr class revolutionised passenger travel, and as part of a national commitment to shaving a day off the transcontinental express the WAGR introduced The Westland in 1938.

[2] The use of Pr class locomotives helped to reduce travel times across the West Australian stage of the journey by more than two hours, accompanied by an increase in the permissible load of 300 tons by additional 20.

The introduction in 1949 of thirty-five Pm and Pmr class was originally intended to oust the Pr locomotives from express services the later designs proved unstable at speed and were transferred instead to fast goods workings, leaving the Pr class as the only express locomotive on the WAGR.

[9] When introduced the P class locomotives were painted in overall black with red buffer beams, in keeping with WAGR livery policies of the time.