The Caledonian Railway 956 Class were 3-cylinder 4-6-0

4-6-0

Under the Whyte notation for the classification of steam locomotives, 4-6-0 represents the wheel arrangement of four leading wheels on two axles in a leading truck, six powered and coupled driving wheels on three axles, and no trailing wheels. This wheel arrangement became the second-most popular...

 steam locomotives that were used on the Caledonian Railway

Caledonian Railway

The Caledonian Railway was a major Scottish railway company. It was formed in the early 19th century and it was absorbed almost a century later into the London, Midland and Scottish Railway, in the 1923 railway grouping, by means of the Railways Act 1921...

 from 1921. They were built to the design of William Pickersgill

William Pickersgill

William Pickersgill was born in Nantwich in 1861 and died in Bournemouth on 2 May 1928. He was Chief Mechanical Engineer of the Caledonian Railway from 1914 until Grouping in 1923...

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

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 railway.

Introduction

The class ran to four members:

• 956 (LMS 14800) — allocated to Balornock Shed
• 957 (LMS 14801) — allocated to Carlisle Kingmoor
• 958 (LMS 14802) — allocated to Carlisle Kingmoor
• 959 (LMS 14803) — allocated to Perth
  Perth, Scotland
  Perth is a town and former city and royal burgh in central Scotland. Located on the banks of the River Tay, it is the administrative centre of Perth and Kinross council area and the historic county town of Perthshire...
  
  

The locomotives 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”.

Dimensions

• Taper boiler, maximum diameter 5 in 9 in (1.75 m)
• Cylinder
  Cylinder (engine)
  A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
  
  s (3), all 18.5 inch (470 mm) bore by 26 inch (660 mm) stroke, in line with the bogie centre
• Outside Walschaerts valve gears were applied directly to the outside cylinders and a form of derived motion
  Gresley conjugated valve gear
  The Gresley conjugated valve gear is a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft...
  
   was used for the inside one.

Problems

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.

The valve gear

Valve gear

The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle...

 was insufficiently robust and the valve spindle guides (originally cast iron

Cast iron

Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...

) had to be replaced in cast steel.

Despite the larger size they were capable of no better work that the preceding 60 class

Caledonian Railway 60 Class

The Caledonian Railway 60 Class were 4-6-0 passenger engines designed by William Pickersgill and introduced in 1916. Six were built and all of them passed into LMS ownership in 1923 and into British Railways ownership in 1948....

 which did not have a sparkling reputation themselves.

Modifications

In 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 Walschaerts valve gears for the outside cylinders and Stephenson link motion for the inside one. 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 be possible to add a second eccentric to drive the combination lever.

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

Dashpot

A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring...

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. By the end of 1922 both had been altered again, this time to the same arrangement as 957/958.

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.