Why a 4-6-0 wheel arrangement, and not a 4-6-2 to improve stability at speed?

The following is taken from Wardale’s response to an article by Bob Butrims published in Locomotives International #56 which itself included a response to the artlcle “What Could You Do With £1.7 Million? David Wardale Answers” as published in edition 55 of the same magazine.  Bob’s response included the comments that: “there is the problem with ash disposal (in narrow fireboxes)”, and that “any firebox that goes down between the frames is a nightmare for maintenance when you get boiler leaks down low”:

“Coming to the comments on my proposal, some of them unfortunately show how prejudices might creep in. For example the idea of a high speed 4-6-0 is criticised on the grounds of riding quality. Like Bob, I have travelled many thousands of km on locomotives, both with and without trailing trucks, in a fair number of countries, and certainly the worst riding in a lateral plane were the Chinese QJ Class 2-10-2s. It is not the presence or absence of a trailing truck (which is undesirable non-adhesive weight, not there for stability reasons but due to the large fireboxes necessary in former locomotives), which makes a locomotive stable, but the whole design of its chassis. Thus it is possible to have a perfectly riding 4-6-0 and a badly riding 4-6-2, and practical examples of both certainly existed. Against Bob’s experience one can, for example, point to the GWR King and Castle class 4-6-0s which by all accounts rode like Rolls Royces, so much so that the crews complained of the riding of the Britannia 4-6-2s when they were introduced. And as eminent a person as André Chapelon proposed 4-6-0s for 200 km/h operation, (see book reviews – Ed.), which he was unlikely to have done if they were inherently unstable. Thus experience of imperfectly designed 4-6-0s should not be used to condemn this type as such. The chassis of the proposed 4-6-0 can and would be designed for very stable riding, roller side control of the leading bogie, roller bearings, self-adjusting wedges, and manganese steel liners and mechanical lubrication of the relevant components (or “dry” lowwear materials), enabling riding quality to be maintained from shopping to shopping, as was actually the case with SAR locomotives so fitted.

Concerning comments made on firebox design, with coal firing using the GPCS (Gas Producer Combustion System), there is no need to “clean the fire” in the conventional sense, but merely shaking the grate on the move to discharge ash into the ashpan, for which a narrow firebox between the frames is more suitable than a wide firebox when the latter, as was usual, does not have a self-emptying ashpan (one of the problems with 3450). Higher thermal efficiency translates into reduced fuel consumption, so that the simple dumping of accumulated ash from the ashpan would be required at relatively infrequent intervals. But as explained in my original article, the type of fuel is optional, and the preliminary performance calculations have in fact been made for diesel fuel (gas oil). Although there is currently some difference of opinion regarding the merits of deep narrow fireboxes and shallow wide ones for oil firing, it is the case that in the past the former has been preferred for locomotives specifically designed to burn only oil fuel. Bob would prefer wide ones which “suit what we do very well”, but he has not mentioned the oily deposits, building up to 6 inches in 5,000 km, which have to be laboriously scraped off the lower firebox walls of No. 711 – another maintenance nightmare which would have to faced every 4 days at the level of utilisation of the former New York Central 4-6-4s!

With modern design and construction methods and water treatment, there would be no leakage from the firebox, therefore having a firebox partly between the frames would not give rise to any maintenance inconvenience. A deep narrow firebox is not a matter of going back to the past, but is rather based on the not-negligible advantages of such a design, as recognised by Chapelon.”