Isn't it very rare that we get to hear what the designers were thinking? What we do know is, like a number of other designs, the 56 was very much an update of an existing design rather than a clean sheet of paper. It was based around Rhymney Railway 0-6-2Ts, but updated with GWR boiler and fitting, piston valves and an all new valve gear design and GWR standard wheels etc. There was a glitch in the early days though. The RR locomotives had a longer wheelbase (7'3" + 8'0") than most other Welsh 0-6-2T classes and as a result they were given a generous amount of side play in the wheels. Enough, in fact, that some even had a vertical joint in the coupling rods. The first 56s, however, it seems were turned out with standard GWR clearances, and the result, we are told, was a rash of axlebox trouble.