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 pounds-force (260 kN).

The 500B class were the second-most-powerful non-articulated steam locomotives to operate in Australia, behind the NSWGR D57 4-8-2.

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.

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.

[1] 504 is preserved as a static exhibit at the National Railway Museum, Port Adelaide.