The 520 class were designed and built by the South Australian Railways to provide a powerful passenger locomotive capable of handling passenger turns on secondary lines; although large engines, the 4-8-4 wheel arrangement and 12-wheel bogie tender spread the weight and hence provided a low axle-load.  Class leader No.520 was completed in 1943 and a total of twelve had been commissioned by the end of 1947.

A notable feature of the 520-class was the classic "shark-nose" streamlining which appears to be based on the work of industrial designer Raymond Loewy and similar to his streamline design for the famous Pennsylvania Railroad T1 Duplex locomotives.  The first three locomotives No.520, 521 & 522 featured a somewhat rounded shark-nose including a shroud around the front buffer beam, whereas the streamlining fitted to No.523 -531 was of a sharper design more similar to the contemporary T1's with a grill near the chimney and the practical advantage of leaving the front buffers and auto-coupler exposed.

Preserved loco No.523 was built at Islington Workshops in 1944 and received the name "Essington Lewis" by the South Australian Railways.  It was among the last 520-class engines in service when condemned in August 1969 after 551,955 miles of service.  No. 523 entered the Mile End Railway Museum in November 1969.

The open-air Mile End Railway Museum closed in 1988, being replaced by a new and improved undercover museum at the former Port Dock Station, Port Adelaide.  This facility is now known as the National Railway Museum and here No.523 is prominently displayed within the main exhibit hall, giving enjoyment to students of industrial design and ferroequinologists alike!