The 720 class were built by the South Australian Railways as an improved version of the 710 class locomotives. The cylinders and running gear were broadly similar, but the boiler was enlarged and its pressure raised from 200 psi to 215 psi. The grate area was increased by nearly a third.
The 710 had been fitted with a Franklin C-2 booster engine[1] on the trailing truck and this was repeated for the 720. However the additional weight of the booster had led to axle load problems on the lines laid with lighter 60 lb rail and some had their boosters removed.[2] Partly to reduce this axle load, partly to support the increased weight of the enlarged firebox, the 720 changed from the 710's 2-8-2Mikado wheel arrangement to a 2-8-4Berkshire. The trailing wheels of this bogie were of different sizes, the larger rear wheels being driven by the booster and a smaller wheelset used under the front of the ashpan to give more clearance.
History
The initial order was for five locomotives. They were intended to operate on the lighter broad gauge lines, but after they began to spread rails, they were confined to the heavier lines. A further twelve were built between 1938 and 1943. The final twelve differed in having streamlined valances and boiler lagging tinware. Coal shortages after World War II saw 14 converted to burn oil. All were later converted back to coal burners.
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