Within a few years of the introduction of the A2 class 4-6-0 in 1907, it was clear that increasingly heavy train loads would require a more powerful locomotive on principal main lines.
From as early as 1918, a series of drawings for potential 4-6-2 'Pacific' type locomotives began to emerge from the VR's Locomotive Design Section, some of which were ultimately developed into the 3 cylinder S class heavy Pacific of 1928.
However, plans for a smaller 2 cylinder Pacific, with an axle load below 20 tons to allow operation across the VR mainline network, were put on hold during the 1930s.
This was partly due to the decline in traffic and revenue due to the Great Depression, and partly due to the improved power outputs and efficiency from the A2 locomotives after the application of a series of smokebox design and draughting changes referred to as 'Modified Front End' in the mid-1930s.

By 1943 however, the situation had changed.
There was a massive increase in traffic brought by the advent of World War II, and the A2s were by this point well past their prime.
The VR Locomotive Design Section once again turned their attention to the proposed Pacific replacement.
The addition of a mechanical stoker, the enlarging of the grate from 37 to 42 square feet (3.4 to 3.9 m²) for increased performance and the use of heavy bar-frame construction for increased durability significantly increased the projected weight of the locomotive.
To keep the axle load to 19.5 tons, the design by 1944 had changed from a 4-6-2 'Pacific' to a 4-6-4 'Hudson' wheel arrangement.
Within a few years of the introduction of the A2 class 4-6-0 in 1907, it was clear that increasingly heavy train loads would require a more powerful locomotive on principal main lines. From as early as 1918, a series of drawings for potential 4-6-2 'Pacific' type locomotives began to emerge from the VR's Locomotive Design Section, some of which were ultimately developed into the 3 cylinder S class heavy Pacific of 1928. However, plans for a smaller 2 cylinder Pacific, with an axle load below 20 tons to allow operation across the VR mainline network, were put on hold during the 1930s. This was partly due to the decline in traffic and revenue due to the Great Depression, and partly due to the improved power outputs and efficiency from the A2 locomotives after the application of a series of smokebox design and draughting changes referred to as 'Modified Front End' in the mid-1930s. 
By 1943 however, the situation had changed. There was a massive increase in traffic brought by the advent of World War II, and the A2s were by this point well past their prime. The VR Locomotive Design Section once again turned their attention to the proposed Pacific replacement. The addition of a mechanical stoker, the enlarging of the grate from 37 to 42 square feet (3.4 to 3.9 m²) for increased performance and the use of heavy bar-frame construction for increased durability significantly increased the projected weight of the locomotive. To keep the axle load to 19.5 tons, the design by 1944 had changed from a 4-6-2 'Pacific' to a 4-6-4 'Hudson' wheel arrangement.
Victorian Railways R class