This proposed locomotive appeared on a diagram dated 31 May 1933.This diagram is reproduced on page 86 of Craig Mackey’s book The 57s and 58s. An artist’s impression was also prepared of this design, which suggests that the proposal was reviewed by management. I wrote an article on this locomotive in the Australian Model Railway Magazine for April 1997, page 81, where the artist’s impression was reproduced.
One question that immediately comes to mind is “why was the odd wheel arrangement selected?” In fact, a similar 2-8-8-0 diagram was also prepared at the same time, but this probably had weight distribution problems. The most likely explanation of the 2-6-8-0 wheel arrangement being selected is that during 1931, the US Great Northern Railway converted some 1910 built 2-6-8-0 locomotives into (very large) 2-8-2 locomotives using the original boiler. This was covered in the US Railway trade press at the time, and it is assumed that someone in the NSWGR drawing office saw the article and realised that the reverse operation, using a modified 57 class boiler on a 2-6-8-0 chassis, was possible.
The Great Northern 2-6-8-0 locomotives were their class M-2 and were simple articulated locomotives, having been converted from class M-1 Mallet compounds dating from 1910 only a few years earlier in 1927-29. Several of these M-2s were rebuilt to Class O-7 2-8-2s, but a number continued to operate as simple articulated locomotives on secondary routes until replaced by diesel locomotives.
The NSWGR proposal was to use a modified 57 class boiler mounted on the frames of 55 class locomotives combined into a simple articulated chassis. Both sets of cylinders were to be fitted with smaller diameter cylinder liners, 18-1/4 inches for the leading engine and 19-3/4 inches for the trailing unit, compared to the 22 inches on a 55 class. This arrangement, along with a reduction of the boiler pressure to 180 lbf/sq. in. produced a nominal tractive effort of 56 000 lbf, the same as that of a 57 class, taking into account a reduction due to the limited cut-off on the 57 class.
The estimated weight of the locomotive was 119 tons, with exactly 16 tons on each driving axle and 7 tons on the leading truck. Given that a 57 class weighed around 139 tons, I suspect that this might be an underestimation, given that there were two sets of cylinders and rods. The plate frames of the 55 class would be lighter than the 57 class cast engine bed, but there were two sets of 55 class frames and presumably a substantial cast set of hinges to link the two frames. I feel that this arrangement might be around ten tons heavier than the diagram estimate. The tender is shown as weighing 89 tons, (although the identical 57 class tender is shown as weighing 95 tons) with a reduction to 64 tons on branch lines, which could be achieved by reducing the coal carried to 7 tons and the water carried to 5000 gallons from the original 14 tons and 9000 gallons.
The diagram has a number of inconsistencies. The boiler dimensions listed are those of a standard 57 class boiler, but the diagram shows a much shallower firebox to clear the trailing driving wheels. It is possible to design a successful boiler with a shallow firebox, the British Railways 9F 2-10-0 being an example. But comparing the 9F firebox to that of the 7MT Britannia, the grate area is significantly smaller and the firebox heating surface is reduced. Looking at the boiler as drawn on the 2-6-8-0 diagram, the grate area would be smaller by around ten square feet and the heating surface would be reduced by around fifty square feet compared to the 57 class boiler.
So the 2-6-8-0 diagram can only be regarded as preliminary, since it does not represent a consistent design. This doesn’t mean that a workable design could not be produced, but there is no evidence that any detail design was carried out.
However, the idea of a locomotive that could haul a 57 class load on light track eventually led to the AD60 Beyer Garratt. These were designed with a variable axle load, by adjusting the load carried by the leading and trailing bogies so they were able to operate on branch lines, but when on main lines (after a visit to workshops) they could make use of an increased adhesive weight. There can be little doubt that the Beyer Garratt was a more useful locomotive than a simple articulated locomotive as shown in the diagram.