The Caledonian Railway 956 Class were 3-cylinder

4-6-0 A 4-6-0 steam locomotive, under the Whyte notation for the classification of steam locomotives by wheel arrangement, has four leading wheels on two axles in a leading bogie and six powered and coupled driving wheels on three axles with the abs ...

steam locomotives A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...

that were used on the

Caledonian Railway The Caledonian Railway (CR) was one of the two biggest of the five major Scottish railway companies prior to the 1923 Grouping. It was formed in 1845 with the objective of forming a link between English railways and Glasgow. It progressively ex ...

from 1921. They were built to the design of

William Pickersgill William Pickersgill (1861 – 2 May 1928) was an English railway engineer, and was chief mechanical engineer of the Caledonian Railway from 1914 until Railways Act 1921, Grouping in 1923. He was appointed locomotive superintendent of the Nor ...

. At the time they were the largest design operated by a

Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish ide ...

railway.

Introduction

The class ran to four members: source: BritishSteam

Dimensions

Taper boiler A fire-tube boiler is a type of boiler invented in 1828 by Marc Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes ...

, maximum diameter *

Cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

s (3), all bore by

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

, in line with the bogie centre * As built,

Walschaerts valve gear The Walschaerts valve gear is a type of valve gear used to regulate the flow of steam to the pistons in steam locomotives, invented by Belgium, Belgian railway mechanical engineering, engineer Egide Walschaerts in 1844. The gear is sometimes name ...

was applied directly to the outside cylinders and a form of derived motion was used for the inside one.

Problems

The

locomotives A locomotive is a rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, push–pull operation has become common, and in the pursuit for longer and heavier freight train ...

were large and visually very impressive, but they were not successful and never really got established in traffic. One author has described them as "that monumental flop of all time", another as "ineffectual giants". Draughting was a problem from the beginning, and firemen had a lot of trouble keeping them up to pressure. There were other troubles thought to be a result of too small an ashpan causing choking of the grate. However, the chief problem was the failure of the unusual derived motion design. The valve gear was insufficiently robust and the valve spindle guides (originally

cast iron Cast iron is a class of iron–carbon alloys with a carbon content of more than 2% and silicon content around 1–3%. Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its car ...

) had to be replaced in

cast steel Steel casting is a specialized form of casting involving various types of steel cast to either final/net or near-net shape. Steel castings are used when iron castings cannot deliver enough strength or shock resistance.Oberg, p. 1332 Examples ...

, but this did not address the fundamental shortcomings of the design and the performance of these large locomotives remained woeful. By early 1922 management decided things could not be left alone and in April of that year No.s 957 and 958 were rebuilt to the strange arrangement of

s for the outside cylinders and

Stephenson link motion The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for various kinds of steam engines. It is named after Robert Stephenson but was invented by his employees. ...

for the inside one. At the same time No. 959 was given a modified version of the standard Caledonian equipment but with

dashpot A dashpot, also known as a damper, is a mechanical device that resists motion via viscous damping. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly us ...

s added to absorb stress that was thought to exist at some points. No. 956 was first given a different variation of the standard arrangement, then brought into line with No. 959, but by the end of 1922 both had been altered again, this time to the same mixed gear arrangement as 957/958. It would have been difficult to retrofit Walschaerts gear to the inside cylinder (because there would be no mounting-point for the combination lever) but it seems strange that a modified Walschaerts gear was not used. It would have been possible to add a second eccentric to drive the combination lever. None of the changes seems to have achieved very much. Experienced locomotive engineers have expressed astonishment and disbelief at the "cacophony" that the mixed valve gear gave rise to, and many have stated that the whole idea was a disaster.

Withdrawal

Despite their larger size they were capable of no better work than the preceding 60 class (which did not have a sparkling reputation themselves), and the allocation of power class '5P' by the LMS was hardly deserved. Within a few years they had been relegated to freight traffic.Atkins, C. P. (1976), ''The Scottish 4-6-0 Classes'', Ian Allan, p.58 Being a non-standard class of only four locomotives the LMS showed no interest in rebuilding them further and they were all withdrawn between 1931 and 1936.

References

Bibliography

* {{Caledonian Railway locomotives

956 Year 956 ( CMLVI) was a leap year starting on Tuesday of the Julian calendar. Events By place Byzantine Empire * Summer – Emperor Constantine VII appoints Nikephoros Phokas to commander of the Byzantine field army (''Domestic o ...