The Commonwealth Railways displayed a pragmatic and risk-averse approach to steam locomotive design, electing to copy existing designs that had been proven in operation with state government railways.  Accordingly the standard-gauge Trans-Australia Railway from Port Augusta to Kalgoorlie was operated by copies of successful NSWGR designs, whilst for the narrow-gauge Central Australia Railway to Alice Springs, the Queensland Government Railways "C17" 4-8-0 design was selected for a series of 22 mixed-traffic locomotives.  These locomotives were known as the NM class (N denoting narrow gauge) and were built by Thompsons Foundry of Castlemaine, Victoria.  Preserved locomotive NM25 was built by Thompson Foundry in 1925 as their Builder's Number 51.

The NM class were generally based at the Commonwealth Railway's Quorn locomotive depot and capably handled the "Ghan" and other Central Australia Railway trains through the Pichi Richi Pass to Maree and on to Alice Springs.  However the Commonwealth Railways were early adopters of diesel locomotives, given the disadvantages of operating steam locomotives in arid areas remote from quality steaming coal supplies, and the majority of the NM class were out of service after 1956.  While scrapping started as early as 1958, a small number of NM locomotives were retained as backup for the diesels, especially in times of flood.  NM34 was the last serviceable NM when it made a final run on an enthusiast tour through the Pichi Richi Pass on 8 October 1967, while sister NM25 miraculously survived as a stationary boiler at Port Augusta until 1975.

NM25 was initially displayed at a Homestead Park, Port Augusta.  It was acquired by the Pichi Pichi Railway in the 1990 and stored in their workshops for the next ten years.  NM25 has since been thoroughly rebuilt and restored to operation via Federal Government grant funding and moved under its own power for the first time in nearly 38 years on 11 April 2003.  Restoration was to a very high standard and included an entire new frame as the original was found to be warped.  NM25 now provides the Pichi Richi Railway with authentic motive power for their recreated "Old Ghan" trains from Port Augusta though the Pichi Richi Pass to Quorn.

The Pichi Richi Railway website includes a detailed history of the NM25 Restoration Project.

For a comprehensive account of the Commonwealth Railway's NM class locomotives, readers are directed to the excellent book "Locomotives and Railcars of the Commonwealth Railways" published by the Port Dock Station Railway Museum.

References