Thunder,
With regard to the name chosen, this project was conceived just after the first Gulf War, when RAF Tornadoes were performing so well. Some would argue about this. Perhaps another racehorse name would have been more PC. How about Red Rum?
As for the seemingly abysmal 12% efficiency of steam locomotives, it is all to do the work that is available to be done in the cylinders in expanding superheated steam from boiler pressure down to saturated steam at just a little above atmospheric pressure.
You have asked a difficult question that I am fortunately able answer, but I don't want to blind anyone with science, so those who don't want to know can look away now. If you know any physics and a fair bit about steam locomotives, you should be able to follow this worked example.
My engineering thermodynamics were taught to me in Imperial units, not modern metric units, but if you will bear with me, the answer is still the same. Imagine 1 lb of water being evaporated in a boiler, that is 1/10th of a gallon. I shall base my calculation on a superheated locomotive boiler steaming at 250 psi absolute, or 235 psi gauge pressure, in round figures.
From my little book of steam tables, the enthalpy of superheated steam at 250 psi absolute and 100 degrees Farenheit of superheat is 1371 Btu/lb of steam. If you expand this to saturated steam at atmospheric pressure, the enthalpy becomes 1150.4 Btu/lb. That means that the only energy in the steam that can be usefully used is 1371-1150.4 = 220.6 Btu/lb, or (220.6/1150.4) x 100% = 19%.
No machines are ever 100% efficient and so you have to take into account the pressure drop across the regulator and the pipes and passages that feed the cylinders. Remember about Sir Nigel Gresley's internal streamlining in the A4s? Then there has to be another pressure drop between the cylinders and the blast pipe. So loco engineers usually assume that only 85% of the energy is available, so the efficiency drops to 85% of the 19%, to say about 16%.
A boiler efficiency of 75% in raising the steam, would be very good, with a single pass boiler and superheaters. For comparison, a modern static coal fired boilers has three passes and gets about 85% thermal efficiency. That is 15% of the heat in the coal is lost up the chimney. So really 75% evaporation efficiency is good for a loco.
All this means that the best overall efficiency that you can hope for is 0.75 x 16% = 12%. However there will a further loss of efficiency in the mechanical transmission of that energy to the wheels. Tornado being a 21st Century steam locomotive will have roller bearings to reduce friction losses, however in a steam age locomotive with superheat and a clean boiler, 8% overall would be a safer bet.
Now think about an early 20th century locomotive with no superheat, a boiler pressure of 150 psi, and conventional bearings. You are looking at perhaps 6% efficiency, or less.
Colombo
