The following answer is taken from Wardale's response to Leonard Staghurst whose letter was published in Steam Railway #277 (copied below):
"Unfortunately Mr. Staghurst's comments betray his lack of knowledge, not only of what is possible with steam traction but also of what has already been achieved. Consider the following.
| Item | SAR 26 Class 4-8-4 | 5AT 4-6-0 |
| Type of locomotive | Rebuild of 1950's design | New design |
| Quality of thermodynamic design | Restricted by structural limitations of existing design | State of the art |
| Number of cylinders | 2 | 2 |
| Simple or compound | Simple | Simple |
| Valves/valve gear | Piston/Walschaerts | Piston/Walschaerts |
| Boiler pressure, lb. per sq. in. | 225 | 305 |
| Fuel | Coal | Gas oil or diesel fuel |
| Ash content of fuel | 15% | 0 |
| Fuel calorific value Btu/lb. | 12,000 | 18,400 |
| Engine weight (excluding tender), metric tons | 123 | 80 |
| Maximum indicated h.p. | 5060 (peak of measured i.h.p. curve) | 3460 (calculated) |
| i.h.p. per ton of engine weight | 41.1 | 43.3 |
The above figures, which are amply supported by other data, show eloquently enough that the calculated power capacity of the 5AT is perfectly realistic, and I regard achieving it as a formality (and surpassing it a probability) - even on this planet! Likewise all factors concerning thermal efficiency, except the target combustion efficiency at maximum evaporation, which I have acknowledged will be a difficult problem. But that does not mean impossible, simply that very good engineering will be required to achieve it."