Mr Attewell criticises the choice of 2 cylinders for high speed, but the 2-cylinder locomotive cannot be simply condemned without fully exploring its possibilities. The acceptability of a 2-cylinder engine hinges on the issue of balancing, as I have acknowledged in my article. Although Mr. Attewell invokes Newton's laws to support his case, it is in fact these same laws which show us how a 2-cylinder engine can be made acceptable. Newton tells us that force = mass x acceleration, so (i) the reciprocating masses cause forces and (ii) these forces act on a mass (the locomotive) to produce accelerations (i.e. vibrations). Considering item (i), force is proportional to the mass of the reciprocating parts, which in the BR 5MT are far from being as light as they should be (notwithstanding the LNER type crosshead, a design which is not amenable to lighter construction by using aluminium). A reasonable yardstick is the mass of the reciprocating parts per ton of piston thrust, so let us consider the following.

This illustrates the extent to which the reciprocating parts can be lightened, correspondingly reducing the forces they exert when in motion. But this is only half the story, the other half being the engine-tender drawgear. Using an American-type coupling incorporating a friction-damped radial buffer allows the tender mass to be added to that of the engine in absorbing fore-and-aft forces, which from Newton's equation proportionally reduces the resultant accelerations (vibrations) of the engine, which is the important parameter. Put simply, if we can roughly halve the reciprocating masses and double the mass of the locomotive resisting fore-and-aft forces (by incorporating the mass of the tender) then the balancing problem is solved, eliminating the objections to a 2-cylinder machine for high speed. The key is optimum design of the reciprocating parts, and it can be revealed that the pistons, piston rings, piston rods and piston tail rods for the 5AT are already fully specified, the total mass of this assembly being only 200 lb., with a stress-based fatigue life equal to the full expected life of the locomotive. It would therefore appear that the target of 550 lb. for the total mass of the reciprocating parts per cylinder is going to be bettered.

Mr. Attewell's other arguments in support of 3 cylinders are rather unconvincing. Adhesion is a complex subject - suffice it to say that there is little practical evidence (especially in Britain!) to suggest that 3-cylinder engines have superior adhesion. And at the kind of running speeds the 5AT would normally be used at the damping of combustion gas flow through the boiler tubes produces fairly uniform draught on the fire whatever the number of cylinders, as I can attest from having taken thousands of draught readings. Of much greater importance is to dampen the draught peaks due to over-rapid release of steam from the cylinders, and I would remind Mr. Attewell that this problem destroyed the performance of the 3-cylinder "Duke of Cloucester", but not that of the 2-cylinder Caprotti Class 5's!

Regarding the designations "mixed traffic" and "express" we should not forget that for the larger BR Standards (i.e. those with 6' 2" coupled wheels) "mixed traffic" meant what it said, and certainly included express duties (the Britannias were used on little else). It was the improvement in front-end design which allowed latter-day steam locomotives to become 'maids of all work' - a colleague during my BR days was only partly joking when remarking that for the schedules of the late 1950's only two classes were required - 0-6-0 diesel shunters and 9F's. Even with 1950's design 6' 2" wheels were no barrier to 90 mph running, and with the 5AT they will be no barrier to 100+ mph.

Mr Attewell would like a larger boiler, but what counts most for steam production is the effectiveness of the exhaust and combustion systems, and all a larger boiler would really do is spoil that most important parameter - vision ahead from the cab.

Caprotti valve gear is expensive specialist equipment, so piston valves and Walschaerts valve gear have been chosen because with Porta refinements they give a performance of the same standard and at much lower capital cost. Indicator diagrams from the "Red Devil" - which the 5AT will greatly improve on - have proved that. Can Mr. Attewell produce figures to show that poppet valve gear requires "considerably less" power to drive than Porta-type lightweight piston valves driven by Walschaerts gear running in needle roller bearings?

The problems of obtaining not only type approval but also authorisation for high speed are real, and have been dealt with in the first part of my article. We will only be ready to tackle these issues when a very comprehensive specification is available for discussion with the various authorities concerned with safety. By that time it is hoped that the experience of the A1 project, which is going at least part way along the path that the 5AT will have to tread, will be a good first guide. But here we must be quite clear what the present project is all about - it is to preserve the aesthetic experience of main line steam, something which may be lost in the future if we rely solely on the continued use of heritage steam. Given this over-riding requirement, we cannot tinker with the overall format of the steam locomotive, which is firmly embedded in the human psyche as the "right" form. For this reason a cab-forward design is out, and means must be found to give the required safety case with the conventional layout - this starting with a moderate size of taper boiler, a chimney top well clear of the smokebox, and optimum cab windows.