An iconic image of rail transport during the steam era in South Wales is a 0-6-2 tank engine, often travelling bunker first at the head of a heavily laden coal train. Almost every company, large and small employed them. But what were they and just why were they so successful?
Lets start with the basics. Just what are 0-6-2 tank locomotives? Although there are exceptions, steam locomotives commonly have up to three types of wheels: leading wheels, larger powered driving wheels and trailing wheels. In Britain we count the number wheels (the Americans count axles) so the system always contains even numbers. The X-X-X classification is in fact based on a system devised by American Frederick Methvan Whyte. Apart from steam locomotives Whyte notation is used to describe the wheel arrangement for some electric and internal combustion locomotives, e.g. diesel-hydraulic shunters.
So applying this system to our locomotives in question, an 0-6-2 has no wheels ahead of the driving wheels. There are six driving wheels and two trailing wheels, As an aside, some wheel arrangements are allocated a name. The famous 4-6-2 locomotives are known as Pacifics.
Although 0-6-2 locomotives could be tender engines, they were almost always tanks, hence, 0-6-2T. The reason will become clear as we go along.
The 0-6-2 Tank in South Wales
As David Maidment describes in his book 0-6-2 Tank Classes, (see a review here) the type was introduced by Tom Hurry Riches on the Taff Vale Railway (TVR) in 1885. Built by Kitson, the Class M became the model on which many similar engines were built for nearly all the railways in the South Wales coalfield. But it was not just the overall concept which was copied. Successful individual designs were occasionally replicated across companies. For example, the design of the Rhymney Railway ‘R’ Class built by Robert Stephenson and Co was used for Brecon and Merthyr engines.
Around the same time that Hurry Riches was introducing 0-6-2 tank locomotives on the TVR, the London and North Western Railway started using the type on the Merthyr, Tredegar and Abergavenny Railway and its valley branches in the form of the Webb Coal Tanks.
When grouping created the Big Four in 1922/23 the Great Western Railway inherited many worn out, poorly maintained and obsolete locomotives. They swiftly realised that the way forward in the South Wales valleys was to create its own more modern version of the 0-6-2. David Maidment mentions that the Great Western tasked its drawing office:
‘….hurriedly to prepare a new design based on the more modern 0-6-2Ts specifically for the Cardiff Valley coal traffic to Barry and Cardiff Docks and for passenger traffic on the Barry-Cardiff/Treherbert/Rhymney/Merthyr main lines.’
Maidment, 2020 P. 166
As a result:
“The first drawings of the new 0-6-2T were prepared by August 1924 and were based on the Rhymney ‘R’ with a Swindon standard No.2 boiler, 8in piston valves and Stephenson valve gear.”
Maidment, 2020: p, 166
The result was the iconic GWR 56xx class used all over the system.
(Credit: Patrick Mackie)
Just what made these locomotives so successful? Here are some reasons. As we go through, bear in mind that many of the points will apply to related types, especially the 0-6-0 tanks including the early saddle tanks and later Pannier Tanks, the many variants of which were similarly successful in South Wales.
1. Overall Slow Speeds on Valley Lines
Valley trains generally operated at slow speed, especially hauling coal or returning colliery empties. There was simply no need for large driving wheels or even additional wheels to provide high speed stability.
2. Small Wheels
So if fast speeds are not required, small wheels can be used to advantage. Smaller wheels not only give better acceleration from standstill, they also give much better traction than large wheels Why should this be? When the locomotive cylinders start to move, the motion is eventually delivered as Tractive Effort at the driving wheel rim. The twisting or turning force that causes the wheel to rotate around the axle is known as torque. Torque is divided by the wheel’s radius, so the bigger the wheel, the lower the Tractive Effort for any given torque. Thus a large wheeled engine will deliver less tractive effort than an otherwise similar one but with smaller wheels.
To place this in context lets look at the wheel sizes of three popular types of steam locomotives:
| Locomotive | Driving Wheel Diameter |
| GWR King Class 4-6-2 tender | 6ft 6in |
| BR Standard Class 9F 2-10-0 tender | 5ft 0in |
| GWR Class 56xx 0-6-2 tank | 4ft 7½in |
All three locomotives spent time on valley lines. Yes, including the prestige passenger express Kings. Their experience is instructive! The difference in wheel size is striking. The BR 9F was designed as a heavy freight loco where tractive effort was more important than flat out speed. Hence the greater number of smaller driving wheels than the King. Smallest diameter of the three is our Class 56xx by a clear 4½ inches.
So, what is the King story? In his book about the class, David Maidment describes two Kings being tested in April 1938 hauling iron ore up the Western Valley from Newport Docks to Ebbw Vale steelworks. These freight workings had been headed and tailed by Churchward Class 42xx 2-8-0 tanks (which incidentally used the same smaller 4ft 7½in wheel diameter as the Class 56xx). King designer Charles Collet had considered the possibility of designing a 2-10-2T to do this work but was stopped by the GWR board.
Maidment described the outcome:
“However, both engines were plagued by severe slipping on the steep gradients in the typically damp Welsh weather and on the second test, 6007 failed to maintain boiler pressure — both engines were being worked at 50 per cent cut-off.
Despite their reputation for good adhesion, their driving wheels were too large for this type of work, and the 42xx and 72xx 2-8-2Ts continued to operate the ore trains until the advent of the BR Standard 9F 2-10-0s.”
Maidment,2020, p. 67
3. Weight of Water in the Tanks
Quite simply, for a tender engine the weight of the water for the locomotive boiler is in a separate vehicle which adds nothing to the weight of the engine itself. Why is this important? Something called Adhesive Weight, which is that part of a locomotive’s weight which is supported by its driving wheels. We have already seen that smaller wheels deliver a greater tractive effort than larger wheels. The extra adhesive weight available to a tank engine allows more of that tractive effort to be applied to the track. The 5AT website makes this point and includes diesels in its consideration:
“In the case of modern Bo-Bo and Co-Co diesels and electrics, 100% of their weight is supported by their driving (or driven) wheels which allows them to deliver a very large starting Tractive Effort. The same applies to 0-6-0T steam engine, however with 0-6-0 tender engines, perhaps no more than 70% of its total weight may be supported by its drivers.”
5AT.co.uk
There is, however, a trade off. The greater the number of wheels, the less weight is available per wheel for adhesion. In the case of a typical 4-6-2 express locomotive less than 50% of total weight may be available for adhesion via the driving wheels.
4. Trailing Wheels
As we have seen the 2 in 0-6-2 means two trailing wheels. The main reason they are there is to provide extra support for the bunker with its load of coal. But these wheels offer another very useful feature. At this point it is important to understand that most engines in the South Wales valleys operated bunker first down valley. This meant that more of the water was covering the firebox, especially important if a locomotive had failed to stop at a water tank!
But this also meant that going downhill with a train full of heavy wagons the two small trailing wheels were now leading. On a train with high inertia, these now helped to guide the train around frequently tight curvature and over points. They also assisted the locomotive to navigate any difficulties the track threw up on sections destabilised by mining.
5 Distance and Turntables
So far we have considered the properties of steam locomotives themselves. But two physical characteristic of South Wales valley lines are also important. Firstly the distance from port to pit and back again was rarely more than 50 miles, negating the need for separate tenders with large quantities of coal. Secondly, away from the major coastal depots and stations, there were very few turntables in the Welsh valleys. In this circumstance an engine without the visual obstruction of a tender and capable of travelling backwards equally as well as forwards had advantages.
But this is not the whole story
Many factors contributed to the success of 0-6-2 tank locomotives, which were deployed by almost every company big and small. Often, companies shamelessly stole plans from their rivals! Among the factors we have looked at are gradient, adhesion on an often wet slippery track and tight track curvature. But of course there were other successful classes. The enormously effective GWR Pannier tank classes were not fitted with non-driving wheels. Similarly the LNWR/LMS operated their G2 Super Ds with much success even though they broke many of our rules by being 0-8-0 tender locomotives. Yet others with similar characteristics such as 0-8-4 tanks were largely failures. But that can wait for another post!
References
5AT.co.uk (2021) Adhesion Available at: https://advanced-steam.org/5at/technical-terms/steam-loco-definitions/adhesion-2/ (Accessed 14/11/2023)
Maidment, D. (2020) Great Western, 0-6-2 Tank Classes: Absorbed and Swindon Designed Classes Pen and Sword Books
Maidment, D. (2020) Great Western King Class 4-6-0s: From Construction to Withdrawal Pen and Sword Books