| Builder | North British Locomotive Co |
| Coupled Wheel Dia | 4'6" |
| Total Weight | 109t 1c |
| Tractive Effort | 25,855lb |
| Coal Capacity | 8 tons |
| Water Capacity | 4000 gallons |
| Number in Service | 35 |
After the second world war the Western Australian Government Railways was
in a rundown state. Many of the older locomotives were basically worn out
and much essential maintenance had been deferred during the war years.
Plans had been made for the construction of ten
Pr class locomotives at
Midland Workshops and for twenty-five improved Pr class locomotives to be
ordered from the United Kingdom. Midland Workshops however was unable to
undertake new construction due to amount of essential maintenance. Thirty
five locomotives were therefore ordered from the North British Locomotive
Company to become the Pm class. The last sixteen were built with roller bearings
on the driving axles and subclassified as Pmr.
The Pm class differed from th eearlier Pr class with provision of roller
bearings on the carrying wheels on the Pm and on all axles on the Pmr class
as well as improvements to the cylinders and provision of steam powered reversing
gear. A departure from standard WAGR practice was the decision to have
independently sprung driving wheels. On the road the independent spriging
was not a success. The Pm class was not able to keep to express passenger
speeds due to rough riding as well as problems with poor adhesion on starting
due to weight transfer. Modification to compensated springing on the coupled
wheels did not offer any improvement. The class was therefore relegated to
hauling fast goods and perishable traffic. The locomotives lasted until the
end of steam operation in 1971.
In addition to Pm 701 preserved at Bassendean the Hotham Valley Tourist Railway
has restored Pm 706 to working order for use on tour trains.