"Challenges for the detail design work for the 5AT
- The only steam locomotives required to operate (transitorily) at 100 mph to maintain schedules were the Milwaukee Hiawatha 4-4-2's and 4-6-4's, and perhaps the DR 05 4-6-4's. These and a few other classes have reached transitory speeds of ~125 mph on test. The 5AT is being designed for continuous 125 mph capability with a 112.5 mph (180 km/h) continuous operational speed.
- The highest thermal efficiency referred to cylinder output of any steam locomotive to date appears to be just over 13% (SAR 26 Class - Porta's experimental 4-8-0 was probably higher, but data is not to hand). The 5AT will have over 15% at maximum cylinder power, which is not necessarily the condition for maximum thermal efficiency (this is greater than the efficiency referred to the cylinders at maximum drawbar power given in FDC.1.3.)
- For high-speed operation the power : weight ratio is critical, especially where a large tender has to be carried to give the necessary range on today's railway. The highest power : weight ratio (of the engine only) to date is ~29 kW/tonne (SNCF 240P Class). The 5AT will be 32 kW/tonne.
- FGS locomotives were the least reliable form of traction (see The Red Devil pages 494-496). This manifested itself in an outsize maintenance effort for the traction work done (compare the total workshop + depot maintenance facilities in the UK now with those of the 1950's). In today's environment with today's costs this is unacceptable and the 5AT will need reliability more or less in line with that of modern traction.
- FGS ran in a much less regulated environment than today's, where it was generally accepted that engineering products were safe. In contrast the 5AT will have to satisfy a host of engineering and safety standards and regulations to prove that its safety will match current requirements.
Five critical areas - speed, thermal efficiency, power : weight ratio, reliability and safety, where the 5AT has to be better than anything done before, better than the engines of Gresley, Stanier, Chapelon, Wagner, Porta et al. The improvements might not seem very great but as % increases over the existing figures they are significant, and any extra would add still further to design complexity (refer to Fig. 163 on page 509 of The Red Devil for the effect of the law of diminishing returns and be thankful that we are not considering a four-cylinder compound, but note that whilst the 5AT is simple in concept it may not be so in detail, this being unavoidable for the level of performance aimed at).
The design has to start from where FGS left off. One cannot skip a generation in engineering development with any certainty as to the outcome, and the outcome must be certain. Therefore the 5AT team has to produce a locomotive from the existing base which will be significantly better than anything produced from that base before. What is more, the success in achieving this will depend absolutely on the detail design of its components: if not designed properly they will not function properly, and as the locomotive is the sum of its components neither will the complete machine. For it to work both the concept and the details have to be right: the concept is determined by the FDC's, the details by the quality of detail design, which therefore has to be better than anything done previously on steam traction."