The 520 class is one of the fastest Australian-built steam locomotives, with engine 520 holding a verified speed record of over 78mph on the Port Pirie line on the 10th of November, 1943.

[1] During the war years in the early 1940s, the South Australian Railways (SAR) had a desperate need for additional tractive power on increasingly growing troop and supply trains and with the combined need for quick acceleration and high speed running on the flat and generally straight mainlines to the north of Adelaide to Port Pirie, as well as power "under the belt" for the long 19-mile (31 km), 1-in-45 (2.2%) graded slog up the Adelaide Hills to Melbourne, a new locomotive design was required by the SAR.

[2] The SAR 520 class was notably featured in the 2016 film Thomas & Friends: The Great Race, as an engine known as "Shane".

[5] Classleader 520 attained a speed of 78 miles per hour (126 km/h) between Red Hill and Port Pirie when it entered service on 10 November 1943.

The long boiler tubes of the 520 class required the blower to be on while in yards, sidings and stations, and while drifting, to prevent blowback or entry of smoke into the almost entirely closed cab.