'' in decorated in green and silver for the Centenary Train which toured the state during the Centenary of
South Australia in 1936 The completion of the
South Australian Railway (SAR)
broad gauge route between
Adelaide and
Port Pirie created a need for a fast, light passenger locomotive to haul this service, as well as other traffic on the lightly laid rail branch lines of the SAR. The specification included the ability to haul a train up a 1-in-45 (2.2%) grade at . Fred Shea, Chief Mechanical Engineer of the SAR, designed a Pacific type. A notable feature of the design, unique to South Australian Railways, was the use of
Baker valve gear in lieu of the more common
Walschaerts valve gear. The first locomotive was completed at the
Islington Railway Workshops in 1936; the last in 1938.
Class leader 620 was also notable for being Australia's first
streamlined locomotive, the sloping smokebox being covered with a chromed steel grille similar to those fitted to motor cars of the period and The press of the time described 620's appearance as "a bit of fluff". The remainder of the class was unstreamlined. In service, the 620 class replaced the Q and
S classes on branchline services, where they proved quite successful. With the introduction of the more powerful and modern
520 class from 1943, these locomotives were relegated to the
Willunga,
Bridgewater and
Tailem Bend passenger services, as well as continued service on
Murray Mallee line services (the locomotives' light axle loading providing good route ability plus the "get up and go" characteristics common to large wheeled Pacific type locomotives. The 620 class was phased out in favour of
Bluebird railcars, although a spate of railcar failures in 1954–55 saw the 620 class return to service on the Port Pirie line. All were withdrawn between 1964 and 1969. ==Preservation==