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 and heavy 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.  (Reference to photos for sister loco No.523 makes for interesting visual comparison of the two streamlining designs.)

Class leader No.520 was built at Islington Workshops in 1943 and received the name "Sir Malcolm Barclay-Harvey".  It was among the last 520-class engines in service when condemned in August 1969. While most of the remaining 520-class engines were scrapped in 1969, No.520 and 526 were retained for enthusiast & historical train duties and No. 523 went to the Mile End Railway Museum.  Alas No.526 was scrapped in 1971 but No.520 was returned to service in 1972 wearing an attractive green livery with gold lining. It was extensively used by the Australian Railway Historical Society (S. A. Division) on tour train duties throughout South Australia during the 1970's and 1980's.