GWR/WR 15xx Class (1948)
I was mulling over the design of the (to me at least) strangely appealing 1948 15xx. It was a pure GWR design, and it appears from the NRM drawings list that it was actually on the drawing boards as early as 1944. As Cook tells us it was designed as a "24 hour shunter", not needing to be serviced over a pit: a worthy aim, but rendered largely obsolete by the early 350HP diesel shunters that were being introduced at the same time. I've seen an interesting comparison made between the GWR 0-6-0PTs and their theoretical equivalents on other lines, the Riddles Austerity/J94, the LMS Jinty and the USA tanks used on the Southern. The numbers indicate that the GWR locomotives have considerably greater boiler capacity than the others, but a similar tractive effort. A pure shunter doesn't really need much boiler capacity, since there is plenty of time for boiler pressure to recover, whereas a locomotive used for traffic work does need continuous steam, and 57s and 94s were regularly used on branch and even short trip main line services.
A flaw/feature in the 15xx design is commonly held to be the relatively short wheelbase, which is reported as rendering them somewhat unstable at speed, and it seems they rarely if ever undertook the traffic roles of other pannier tanks, although the survivor with the Severn Valley seems to do well enough at preserved line speeds. The actual wheelbase is 6ft 4in + 6ft 6in, 12ft 10in. Its interesting to compare this with dedicated short wheelbase dock shunters, such as the GWR 1361 and 1366 classes , 6ft + 5ft - 11ft, , the USATC S100 at 5ft + 5ft for 10ft and the Riddles Austerity 5ft 9in + 5ft 3in for 11ft. The shortest wheelbase regular 0-6-0T on the GWR was the 850 class, 7ft 4in + 6ft 4in - 13ft 8in. The short wheelbase on the 15s is commonly held to be intended to improve their ability to traverse curves, and their work in Newport and on the Paddington ECS workings stated to support this. Its interesting that the wheelbase on the 15s is intermediate between the pure shunting types listed above and traffic locomotives such as the 850, and even more the other large pannier tanks, 94xx, 57xx and their 7ft 3in + 8ft 3in 15ft 6in wheelbase. As such it has occurred to me that the 15xx wheelbase might be for other reasons than curves. My theory is this: the 57 and 94 cylinders are set partially between the wheels, as is possible with inside cylinders. The big outside cylinders on the 15 can't be, so the leading wheels have to be set back relative to the smokebox in order to clear the cylinders. In addition, whereas on the inside cylinder locomotives the cylinders themselves brace the frame, on the 15xx there's a large structure between the cylinders to perform the same function. All this makes the locomotive heavy, and in particular front heavy. This in turn means that the trailing wheels have to be set forward for the locomotive to balance. The 15 is heaviest on the leading wheels and lightest on the 3rd pair, whereas the 94 is opposite. It would be interesting to know what someone better versed than I on the subtleties of steam locomotive designs makes of that idea.
Its often claimed that the 15xx was inspired by the S100/USATC 0-6-0T. The locomotives are indeed superficially similar, with prominent outside cylinders, outside walschaerts valve gear, external steam pipes, water tanks that do not flank the smokebox and no footplate. However this claim doesn't appear in any of the memoirs of contemporary GWR staff that I am familiar with. Given the design aim declared by Cook, a 24 hour shunting locomotive that did not need to go over a pit for servicing, then when examined in detail the comparison is less certain. The design aim forces outside valve gear and outside cylinders. GWR practice was to use walschaerts gear on (their few) outside valve gear locomotives - notably the railmotors and the VOR 2-6-2Ts. By this time external steam pipes were standard on GWR outside cylinder classes. The S100, with its very short (10ft) wheelbase drives to and has the eccentric on the rearmost driving wheel. Apart from anything else there would be no room for the valve gear driving on the middle wheel. The 15xx, on the other hand, has 12ft 10in wheelbase, and a connecting rod driving the trailing wheels would be some 13ft 6in long. The longest connecting rod on any of the Churchward standards was 10ft 8½ inches. I wonder if 13ft 6in would be practical. Here's a list of similarities and differences.
Similarities
No footplate
Outside cylinders with prominent steam pipes
Outside Walschaerts gear
Wheel size 4'6 v 4'7.5
Differences
coal capacity (1 ton , 3.25 ton)
parallel/taper boiler
driven wheel
wheelbase (s100 as short as possible, 15xx longer)
boiler proportions (much bigger boiler on the 15xx)
All in all, I submit that there's a strong case to describe it as convergent evolution, rather than consider the 15xx to be a direct descendant of the S100. Arguably the only feature of the 15xx which may not be extrapolated from previous GWR practice is the absence of footplate. This could well be a weight saving feature, and in that respect we might also look at Bulleid's 1942 Q1 as an inspiration.
On the other hand the GWR drawing office must have had drawings for the S100 available, since the first weight diagram for the type at Swindon is dated July 1943. They may have been in service at WD sites adjacent to the GWR as early as 1942, but RCTS states they were not used on GWR metals until June 1944. The first drawings at the NRM for what became the 15xx are dated February 1944.
Edited by JimC
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