Caledonian Railway 956 Class

The Caledonian Railway 956 Class were 3-cylinder 4-6-0 steam locomotives that were used on the Caledonian Railway from 1921. They were built to the design of William Pickersgill. At the time they were the largest design operated by a Scottish railway.

Introduction

The class ran to four members:
* 956 — allocated to Balornock Shed (LMS No 14800)
* 957 — allocated to Carlisle Kingmoor
* 958 — allocated to Carlisle Kingmoor
* 959 — allocated to Perth

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 feet 9 inches (1.8 m)
* Grate area, 28 square feet (3 m²)
* Working pressure, 180 lbf/in² (1.2 MPa)
* Cylinders (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 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 was insufficiently robust and the valve spindle guides (originally cast iron) had to be replaced in cast steel.

Despite the larger size they were capable of no better work that the preceding 60 class 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 retro-fit 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 dashpots 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.

Withdrawal

They were all withdrawn by 1935.