South Australian Railways 500 class (steam)

The South Australian Railways 500 class was a class of 4-8-2 steam locomotives operated by the South Australian Railways. The locomotives were rebuilt in 1928, when booster engines were installed, becoming the 500B class with a wheel arrangement of 4-8-4 and a tractive effort of 59,000 lbf. The 500B class were the second-most-powerful non-articulated steam locomotives to operate in Australia, behind the NSWGR D57 4-8-2.

History
The 500 class were part of larger order for 30 steam locomotives placed with Armstrong Whitworth, England, in 1924, as part of the rehabilitation of the state's rail system being overseen by Railways Commissioner William Webb. They replaced the Rx and S class locomotives, many dating back to 1894, that were still performing mainline duties, meaning that double and even triple heading was common. All ten 500-class locomotives arrived in Adelaide in 1926, and entered service on the Adelaide to Wolseley line as far as Tailem Bend. All were named after notable South Australians.

Rebuilding
In May 1928, 506 was experimentally fitted with a booster in a newly built four-wheel trailing truck, replacing the original unpowered two-wheel truck. The modification, popular in the United States, proved highly successful, increasing the locomotive's tractive effort from 51,000 lbf to 59,000 lbf. Boosters were subsequently fitted to the nine remaining locomotives, resulting in the class changing to 500B – although their popular name, among railway employees and the public alike, remained "Mountain class" rather than their new "Northern" configuration. With their upgraded tractive effort, the 500B class briefly became the most powerful non-articulated steam locomotives to operate in Australia until supplanted the following year by the New South Wales D57 class.

Throughout the mid-1930s, all but two of the locomotives in the class were semi-streamlined and had valances fitted. The first two locomotives were withdrawn from service in 1955 and the last was withdrawn in 1962.

504 is preserved as a static exhibit at the National Railway Museum, Port Adelaide.