The class 620 was a Pacific designed for passenger service on light rails with only 60 pounds per yard or 30 kg per metre. Ten were built in the Islington Railway Workshops starting in 1936 after the broad gauge line between Adelaide and Port Pirie was completed. The requirements said that they had to be able to haul a train of 200 long tons on an incline of 1 to 45 (2.2%) at 25 mph or 40 km/h.

The locomotives weighed a little over 80 tons in working order or 140 tons with a four-axle tender. Contrary to previous locomotives, the valve gear was of the Baker type. Only No. 620, the class leader, was streamlined and had a chromed steel grille. It was painted in a mix of Hawthorn Green and silver and was named “Sir Winston Dugan”. Numbers 621 to 629 were not streamlined, but had smoke deflectors which were integrated into the shape of the boiler.

They were not only used for pure passenger trains, but also for mixed trains which included some freight cars, but were faster than pure freight trains. From 1943, they got competitors in the 4-8-4 class 520. Ultimately, they were replaced by Bluebird railcars, although these suffered from teething issues and needed some time until they were completely ready for service. Finally, the class 620 was withdrawn between 1964 and 1969. Today 621 is in operational condition and 624 is on static display.