|Introduced||1937 – 1939|
|Weight – Loco||68t 8cwt|
|Driving Wheels||5ft 2ins|
|Boiler Pressure||200psi superheated|
|Cylinders||Three – 18½in x 26in|
|Valve Gear||Walschaert with derived motion (piston valve)|
In 1937-1939 Gresley introduced his first three-cylinder K4 class specifically for working the North British Railway (NBR) West Highland line to Mallaig via Fort William. This line presented many operating problems due to its steep gradients and severe curves, combined with very restrictive axle loading limits. Although they were nominally the most powerful of Gresley’s moguls, with their small 5ˊ 2˝ diameter wheels, they only had a relatively small sized boiler. This was because they only needed to use their high power for short periods at low speed on the West Highland line.
At Grouping in 1923, passenger services on the West Highland line were being hauled by D34 Glen 4-4-0s due to the axle loading restrictions. Heavier stock had already entered service, and was already resulting in some double-heading of the D34s. An early proposal was to use the new K3s to provide extra power, but they would have been restricted from the Mallaig section of the line, and was vetoed completely by the Civil Engineer after K3 bridge tests were made.
|D34 Glen class introduced by Reid on the NBR in 1913|
|K3 introduced on the GNR by Gresley in 1920|
|K2 introduced by Gresley in 1912.|
|K2 as rebuilt with side windows for working in Scotland|
By October 1924, a proposal appeared for a new 2-6-0 with 5ft 2in coupled wheels, a K3 boiler, and three 18in by 26in cylinders. This proposal was dropped when the Great Northern (GN) section loaned fourteen K2s for use on the line. These provided satisfactory, providing an improvement in power and adhesion. Also with K3s displacing the K2s elsewhere on the LNER network, new work was required for the K2s and the loan became permanent in October 1925.
There is some evidence that a 2-8-0 with 5ft 2in driving wheels was considered in 1934. This would have combined the required adhesion with low axle loadings, but was never developed further.
In September 1934, Gresley instructed Doncaster to investigate the possibility of increasing the tractive effort of the K2s by altering the boiler pressure and/or the cylinder diameter. Calculations showed that by increasing the boiler pressure to 220psi and the cylinder diameter to 21in, the tractive effort would be increased to 31,351lbf. This would have resulted in a reduction in the factor of adhesion to 3.7, which Doncaster recommended against.
Provision for one locomotive was made in 1935 by the Joint Traffic & Locomotive Committee, but work on the new design did not start until about May 1936. When design work finally started, it proceeded very quickly with an outline drawing appearing in July, and signing of the construction order in September. This design was based on the 1924 proposal for a 2-6-0 with 5ft 2in diameter coupled wheels, but with K3 cylinders, a K2 boiler, and a B17 firebox. The boiler pressure was set at 180psi to provide a tractive effort of 32,939 lbf, and an estimated factor of adhesion of 3.92. The frame was 5in longer than the K3 frame, and a similar pony truck was used although the K4 pony truck had a 6ft 7in radius swing compared to the K3’s 5ft 11.5in radius swing. Authority for the new engine had not been included in the 1936 Build Programme, so a pending order for twenty-one K3s was reduced to twenty.
The prototype K4 3441 (61993) Loch Long was completed at Darlington in January 1937 and was allocated to Eastfield depot, Glasgow. The locomotive and was virtually identical to the July 1936 outline. The final factor of adhesion was 3.94 due to a slightly heavier locomotive weight, adhesive weight, and axle loading. The new locomotive quickly showed itself capable of hauling 300 ton loads to Fort William without assistance but the 180 psi boiler pressure brought little improvement in average speeds over the existing K2, and that 3441 responded sluggishly when up against the gradients of the West Highland line.
Gresley reacted by raising the steam pressure to 200 psi which saw the tractive effort leap to 36,598 lbf, with a corresponding reduction in the factor of adhesion to 3.54. The K4 could now demonstrate its true capabilities handling 300 ton trains and with maximum speeds around 60 mph on level ground. An advantage of the newcomer was that it used only marginally more coal in working 300 ton trains than the K2s did with considerably lighter loads.
The successful trials with 3441 led to five more being ordered in February 1938, and these entered service between July 1938 and January 1939. Due to their design being specifically for a line with very specialised requirements, no further K4s were built.
Apart from the prototype Loch Long all were named after Highland chieftains and grandees.
Between 1937 and 1940, all six K4s were adapted to take small snowploughs. The conversion consisted of a bracket fitted to the running plate above the draw-hook, and holes along the bottom of the buffer beam.
In 1945 Thompson rebuilt 3445 (61997) with two cylinders and it was reclassified as class K1 (later K1/1). It was used later as the prototype for the Peppercorn K1 class.
Spark arresters were fitted to the remaining K4s in April and May 1947.
Initially the K4s were mainly used on the Glasgow to Fort William stretch of the West Highland line. They handled the gradients and continuous curves with ease, but they were not designed for the 20 miles of level track near Glasgow, and the 8.5 miles stretch alongside Loch Eil. Vibration at speed was a problem on these stretches, and the middle big-end would require regular nut tightening. On one occasion, the middle connecting rod actually dropped off. As a result of these issues maintenance costs increased. The V4s with their slightly larger wheels were more popular with the enginemen but were lacking in power. Eventually the Thompson B1s appeared on the Glasgow to Fort William services from 1947, displacing the K4s to the Mallaig stretch. The new Thompson/Peppercorn K1s appeared in 1949 and were preferable due to their lack of a troublesome middle big-end. After Nationalisation, LMS Black 5 4-6-0s were also moved to the line. From this point onwards, the K4s were concentrated at Eastfield and intended for use on West Highland goods trains only.
|B1 class introduced by Thompson in 1942|
|V4 class introduced by Gresley in 1941. Only two were built.|
|K1 class introduced by Peppercorn in 1949|
|LMS black five introduced by Stanier in 1934|
During the 1950s, the K4s’ sphere of operation enlarged and they began to appear at locations such as Edinburgh, Perth, Forfar, Ayr and Tweedmouth.
In December 1959, the five K4s were transferred to Fife (two to Thornton Junction and three to Dunfermline Upper) for use on goods trains until October 1961 when four were withdrawn. The last K4, 61994 was withdrawn in December 1961.