Almost the whole of Herr Ebel's technical criticism is based on experience with former German locomotives, the implication being that this somehow places a barrier against further development and that consequently the performance figures given for my proposal must be wrong. Common sense should tell us this is false. Look at other engineering products of the 1950's, when the last main line German steam locomotives were being built, and compare them with to-day's. Even something as basic as the bicycle has seen significant technical advances during those forty-plus years, but it seems that the possibility of similar progress is being denied to steam traction, as though it alone defies the law of technological change. This thinking is wrong, and it is precisely because significant developments have been made in steam locomotive design that my performance figures are both well in advance of those formerly achieved yet also perfectly realistic.

The key to achieving this lies primarily in the work of L. D. Porta, building on that of A. Chapelon. Porta has shown that nowhere did steam locomotive design reach a peak, on the contrary, it fell well short of what could have been achieved with the Stephensonian locomotive. Porta's genius lies in taking each component of the locomotive, down to the humblest nut and bolt, getting a thorough understanding of its principles, both in isolation and in relation to the locomotive as a whole, and then optimizing it by improved design. Another lesson is his holistic view of the locomotive and the consequent doctrine of using all reasonable methods to achieve a desired goal, methods which together can act synergistically to lift performance onto an altogether higher level. Here it must be said that those who are not widely conversant with Porta's technology do not know the extent of the improvements which it can effect to steam traction, nor how they can be achieved.

Let us take Her Ebel's points item by item. He dismisses the concept of a high power 4-6-0, claiming that such locomotives were not built world-wide for heavy duty since 1920. This is incorrect, as, for example, the Great Western Railway built nothing other than 4-6-0's for its heaviest express passenger services until it was nationalized in 1948. It is also relevant to point out that for ultra high speed (200 km/h, which naturally requires high power) Chapelon proposed 4-6-0's, both in 1938 and 1943.

The reason for condemning the 4-6-0 is its supposedly inadequate boiler capacity. Even in the past this was not necessarily true, for, as Chapelon has pointed out, size for size a boiler with a deep firebox, as generally used on 4-6-0's, has greater capacity than a shallow firebox boiler. And it is certainly not true now. Consider what Porta has called the cardinal equation of the steam locomotive: {power = evaporation / specific steam consumption (s.s.c.)}. From this equation it is clear that reducing the s.s.c. increases the tractive power which can be obtained from a given boiler, and the 4-6-0 proposal would use the considerable number of means available to guarantee high cylinder efficiency and very low s.s.c. Calculations point to an indicated s.s.c. of 1,88 kg/MJ (based on steam to the cylinders only) at the operating conditions stated in the original article, i.e. 130 km/h and 2 500 hp at the drawbar.

Coming to the numerator of the equation, evaporation, it should be understood that this largely depends on the exhaust and combustion systems. "Boiler stress", much used in Germany, is not a good measure of boiler capacity, unless heating surface area and the more important criteria of grate area for coal firing or firebox volume for oil, and especially the mean free gas area through the tubes, are closely related to each other (which may have been the case for the various Reichsbahn boilers). This is because boiler stress relates output to heating surface and therefore to heat transfer, which is far less critical to steam production than heat generation, this in turn depending on the ability of the exhaust to pump combustion gas through the boiler at the required rate. To quote from Chapelon, "the evaporative heating surface, to which such great importance has been attached for so long, is nevertheless only a secondary factor in the organic functioning of the locomotive."1 German experience itself illustrates this, for in actual service were welded boilers of similar size not rated quite differently depending on whether they were coal or oil fired? But of course boiler heat transfer is not to be neglected, and the 4-6-0 will be designed to achieve a boiler absorption efficiency of 80-85% at full load.

The layout of the boiler tube bundle in the 4-6-0 would be decided at the detail design stage, but the combined (tube + firebox) evaporative heating surface area will be approximately 160 m², well within that possible in a 4-6-0 boiler. For an estimated total evaporation of 18 600 kg/h this means a boiler stress of 116 kg m^-2h^-1, or not much greater than the 100 kg m^-2h^-1 claimed for the 0110's. We can mention that Chapelon's coal-fired 240-700 and 240P Class 4-8-0's exceeded 100 kg m^-2h^-1, the SAR 26 Class 4-8-4 burning poor coal with no caking properties reached 110 kg m^-2h^-1, and in Argentina 140 kg m^-2h^-1 has been reached with oil firing.

But, I repeat, the critical question is not how much steam can be produced per unit of heating surface, but rather how to generate the necessary heat in the firebox.. Whilst coal firing is an option (in which case the GPCS is the only way the required performance may be achieved with lump coal) the preferred fuel is gas oil, technically equivalent to diesel fuel, and on which the preliminary performance calculations have been based. Given today's advances in combustion equipment, no difficulty is envisaged in achieving the required heat release rate of about 13 GJ per m3 of firebox volume per hour, within the limit already achieved with oil firing, at high combustion efficiency (for example, the Swiss Sonvico system is said to give some 99% combustion efficiency virtually independently of boiler load). However this depends on sufficient draught being provided by the exhaust. Linking as it does the functions of heat generation in the firebox, heat transfer in the boiler and superheater, and steam utilization in the cylinders, the exhaust is the absolute key to enhanced locomotive performance, a point of cardinal importance. Thanks to Porta's designs we now have draughting systems that are superior to any used before, and which make high evaporation from a given size of boiler a practical proposition.

One fallacy in Herr Ebel's reasoning, which can be deduced from his figures, lies in his assumption of the same s.s.c. for the new locomotive as for the DB 0110 Class. As s.s.c. is an independent variable, boiler size or capacity cannot be related to a specific level of power. There are a whole host of improvements - far too many to list here - which can be made all the way from the throttle to the chimney to improve cylinder efficiency, and therefore the reasoning that an 0110 Class sized boiler would be needed is not correct. It should also be noted that boiler capacity can be significantly increased by combustion air preheating and by a high feedwater temperature, which latter is best achieved by a 'closed' rather than 'open' type heater (in America feedwater heating was probably viewed at least as much as a means to increase power as one to save fuel and water).

To summarize: by optimizing all important criteria, i.e. (i) moderate evaporation due to low s.s.c., (ii) high efficiency combustion of gas oil, (iii) high boiler absorption efficiency, (iv) highly effective draughting, and (v) maximum pre-heat to feedwater and combustion air, the 4-6-0 boiler will be able to supply enough steam for the intended power.

Regarding the mechanical limitations on the locomotive boiler given by Herr Ebel, the proposed working pressure of 21 bar is less than the maximum successfully used in America, and the normal nature of train operation will give a mean boiler stress significantly less than the nominal maximum figure of 116 kg m-2h-1. Experience shows that the mechanical well-being of a boiler is more a question of good detail design (much of which originated in Germany) and water side cleanliness than of pressure or loading. Thus a high pressure boiler designed to accommodate high thermal loading without excessive mechanical stress, and which is kept scale and corrosion free by the kind of water treatment developed by Porta, will give less trouble than a badly-designed boiler operating at low load and pressure but which is allowed to scale up.

Coming now to the engine, nowhere have I advocated four cylinders. Preliminary calculations have been made on the basis of two simple expansion cylinders and show that adhesion is fully utilized on starting with cylinders of 450 mm diameter x 800 mm stroke, and that with 1 880 mm diameter coupled wheels these can deliver the 3 500 indicated hp required at 130 km/h at economical cut-off. That such high power can be obtained from such modest cylinders is a tribute to Porta's technology. The keys lie in the high boiler pressure, in minimizing heat transfer losses, and in the utmost internal streamlining all the way from boiler to chimney (how much could be written about that!), so that the maximum possible fraction of the energy in the steam leaving the superheater is used to drive the train. Indeed it was not always so in the past! No fundamental difficulties are foreseen in the thermodynamic and mechanical design of such an engine, nor in achieving a satisfactory solution regarding absorbing it's inertia forces. My present inclination is therefore towards a two cylinder simple expansion engine, in which case there will be no machinery between the frames to cause all the problems Herr Ebel mentions.

There will be no difficulty in designing the driving gear for the required power. Here it should be noted that the SAR 26 Class 4-8-4 modified by the writer generated 1 500 - 2 000 indicated hp per cylinder in everyday traffic, with no adverse effect on the driving gear, bearings, or frame, whilst in America up to 3 000 indicated hp per cylinder was reliably transmitted. Achieving this is a question of good detail design rather than overly-massive rods. Concerning bearings, the choice of spherical roller bearings (as were used in Germany) is that they accommodate the deflections and misalignments which occur in rods, crankpins, and axles. Roller bearing selection is a well-developed art, and is much easier for large-wheeled locomotives with moderate piston thrusts than for heavy freight locomotives, so having designed roller bearing rods and axles for the Chinese QJ Class - a most difficult task - during my time at Datong, I do not anticipate any trouble obtaining a reliable design for the case under discussion. Note that roller bearings are now of much greater capacity than forty years ago, this being one example of technological progress which can benefit steam traction.

Spring-loaded or automatic wedges are actually quite simple devices which act to completely eliminate shock loads otherwise arising from axlebox - guide clearances. Such shocks are damaging to the bearings and the frame (therefore eliminating them is one means to prevent the frame and bearing troubles Herr Ebel predicts), and can be responsible for things as diverse as increased tyre wear and leaking pipe joints. The difference in smoothness of running between a locomotive fully equipped with roller bearings and automatic wedges and one not so equipped has to be experienced to be believed, and it lasts from one heavy repair to the next. So automatic wedges are no unnecessary complication, they are the just the kind of detail refinement which can effect a real improvement in locomotive operation, and became standard fittings in America and elsewhere.

Finally on the subject of the mechanism, it should be noted that needle roller bearings were an integral part of the American Baker valve gear, and are extensively used on Walschaerts valve gear on Chinese locomotives, operating successfully in a rather harsh environment.

Regarding adhesive weight, it is surprising to read that no more than 54 tons can be expected from a 4-6-0. The BR Standard Class 5, on which my proposal is based and which is considered in Britain to be only a medium-size locomotive, has a maximum axle load of 20 tons and 58 tons adhesive weight. Several British 4-6-0 classes exceeded 60 tons adhesive weight, the heaviest being the Great Western Railway "King" Class with 68,6 tons. It can be noted that the "Kings" were lighter than all "standard" German classes of 4-6-2, yet exceeded all in adhesive weight, and that their total evaporative heating surface area was only 14% less than that of the DB 10 Class. Their grate area was naturally somewhat smaller (it could afford to be, because of the excellent Welsh coal they burnt), and we should recall that 4-6-0's developed into 4-6-2's primarily to get a larger grate area, otherwise they would have been better as 4-8-0's.

Coming now to more general points, Herr Ebel seems to be under some confusion as to the services my proposal would be used for. As stated in my article, although it could be used on museum railways the 4-6-0 is intended for main line specials, a market which will increase if charter trains become more popular (it is not only trains for railway enthusiasts which need steam traction to provide the right "ambiance"). Also envisaged are "Plandampf" operations by train operating companies of their own regular scheduled trains. Given this, Herr Ebel's points about the unsuitability of the proposal for purely museum railway use are agreed to, the locomotive not being intended for such service, where it would be grossly underused.

My interest is in main line operations, where steam can be worked hard and fast and really experienced at its best, and I believe it would be wise not to ignore the potential future problems of operating main line steam over the railway network. It is accepted that the situation in Germany may be rather different from that in the UK. Here it is likely that the difficulties which the railway system is currently experiencing will be overcome and that already busy main lines will eventually become so saturated with traffic that little space will be available for special trains operating at lower speed. Steam will then be pushed to secondary lines, which is not necessarily where specials can be most profitably operated. The 4-6-0 proposal aims to provide a ready-made solution to this problem when it arises.

Although Herr Ebel dismisses infrastructure problems, the very means he describes for coaling and watering locomotives shows that these problems do exist. They are worse here, partly because British locomotives carried very small tenders, to maximize train trailing : gross weight ratio, which is so important to locomotive efficiency, especially at high speed. With limited tender water capacity, replenishment on longer runs was from water troughs, which are, of course, no longer there, so that heritage locomotives have to stop relatively frequently for water, a serious operating nuisance.

Herr Ebel's lengthy dismissal of fuel cost as an important factor misses the greater point of striving for higher thermal efficiency. As {output = input x efficiency} high thermal efficiency is necessary for high power. So a more efficient locomotive can be operated either to save fuel or at greater power, i.e. at higher speeds on heavier trains. Although a modern steam locomotive would indeed use significantly less fuel than one of the 1950's to deliver the same power (just as, size for size, to-day's cars do) its ability to generate the higher power necessary to keep up with all the other trains crowding to-day's main lines would be the decisive factor.

In his last paragraphs Herr Ebel shows his true intention, which is the preservation of the past. It is perfectly acceptable to want to preserve the past in the way he describes, and I sympathize with his view that museum railways should concentrate on that. But this past will simply not be able to work on busy main lines, ever more crowded with trains moving at 160 km/h or more, and it is to that future that we should be looking. I am not dreaming of "my" locomotive (which would be an improved C & O Allegheny 2-6-6-6!) but am presenting this proposal to the enthusiast community to see if the time is right to start working towards a solution to the problems I believe main line steam will face sometime in the not too distant future, to provide a locomotive which is not a recreation of history but which is something new and dynamic, making history now. Whether this is a phantom or not will depend not on engineering but on the attitude of the railway enthusiast community. If like Herr Ebel it prefers to direct its vision backwards it will indeed remain a phantom, if not then it - and the long-term future of main line steam - has a chance.

Lastly, for disbelievers, a tale from the SAR. When presenting proposals for rebuilding a 25NC Class 4-8-4 I was told by SAR engineers that the claim for 36% increase in drawbar power, from 2 500 hp to 3 400 hp, was exaggerated, and a friend on BR who is very knowledgeable about steam locomotive performance said he found it "unconvincing - unless you put in a third cylinder." In fact the drawbar power subsequently sustained on test by the modified locomotive reached some 3 790 hp, the trend pointing to just over 4 000 hp at higher speed than could be tested at, this actually being reached from time to time in passenger service. Indicated powers of up to 4 570 hp were measured, the trend suggesting a peak of over 5 000 hp at higher speed. This was from a narrow gauge locomotive with two 610 mm diameter x 711 mm stroke cylinders, 15,5 bar boiler pressure, burning mediocre coal, and having a maximum permitted speed well below that for optimum cylinder performance. Also, being a rebuild, it had plenty of limitations attributable to the original design. One day, with my friend with me in the cab, I was able to demonstrate to him this new level of power, which he later confirmed from train resistance calculations. He never found a third cylinder on the locomotive, and was afterwards wise enough to stay silent about his previous remark.