GWR Route: Banbury to Wolverhampton

Leamington Spa - Old Station: gwrls897

An unidenitified GWR 4-6-0 King class locomotive at the head of the 9.05 Birkenhead to Paddington express train runs into the up platform. The Kings were a regular performers on the route to Birkenhead from the 1930s through to the end of steam on GWR lines in the early 1960s.

The King class was the final and most powerful development of the GWR’s 4 cylinder designs. GWR CME (Chief Mechanical Engineer) Charles Collett’s Castles were arguably the GWR’s finest passenger engine design, and on a tractive effort basis, had proved themselves more powerful than LNER’s Flying Scotsman. Nevertheless, the advent of the Southern Railway’s King Lord Nelson class as the new leader in British express steam tractive effort (33,500lbs) pushed the GWR’s publicity office to push for one greater engine class, with a nominal tractive effort of 40,000lbs.

The apparent need for such a class existed on the heavily graded routes in South Devon, en route to Plymouth. However the ‘hammer blow’ of the weight of each axle plus the effect of reciprocating parts was expected to be in the region of 22½ tons, 3 tons in excess of the 19½ ton static design limit of most GWR bridges. Luckily, across the huge and unwieldy GWR organisation, the civil engineering division had been independently raising the bridge loadings to 22 tons for the previous 22 years, and a concession of 22½ tons was made for 4-cylinder engines. For 22½ ton axle loading, routes from London to Plymouth and Cardiff, on a handful of bridges would need strengthening. These routes would become the ‘double red’ routes of the GWR, and the Kings would rarely stray from them.

Collett opted for smaller wheels on the King than his Castles, after casting aside the conventional wisdom that large wheel diameter was needed for the greatest speed. He had observed an express train being overhauled by a mineral train hauled by a close-coupled GWR 4-8-0 with small wheels. When compared with the Castle class, the slightly smaller wheels adopted for the King Class allowed more space above them for a fatter boiler to be built, as little extra engine height was available for expansion.

Nevertheless, such a reduction in wheel size was not without its corresponding problems. A long standing Swindon design constraint imposed by former CME George Jackson Churchward, was that the pistons would be level and not slanted. This meant the centre height of the pistons would be the same as the driving wheel axles. The smaller wheels thereby lowered the height of the pistons. At the same time, to achieve the 40,000lb tractive effort, the pistons were enlarged to 16¼” bore and 28” stroke. The outer pair of pistons, acting on the centre wheels, needed to hang either side of the rear wheels of the front bogie. The hidden inner pair of pistons, acting on the front set of driving wheels, hung low between the front wheels of the bogie. Both factors meant a traditional springing arrangement for the bogie was impossible. A compromise design with the front wheels sprung on the outside and the back wheels of the bogie sprung on the inside, produced the distinctive and decidedly odd-looking long front bogie of the Kings.

Like on the Stars and Castles before them, the Walschaerts valve gear was concealed between the frames, and driven off the front set of driving wheels. The two inner valves, directly above the inner pistons, were driven directly from these sets of valve gear. The outer valves, directly above the outer pistons, took a reflected drive from the inner valve gear via a pair of rocker arms emerging from the frames above the front bogie. Like the Stars and Castles, no external valve gear was visible, but this aesthetically pleasing arrangement made for a maintenance nightmare, with over 120 oiling points, many hard to reach, before the engine could take to the road.

Draughting arrangements included a 'jumper' blastpipe ring (the blastpipe is inside the smokebox and directly under the chimney - it directs the steam exhaust straight up into the chimney and in doing so creates a strong sucking action on the fire in the firebox, drawing the hot gases through the tubes of the boiler.) A jumper ring was designed to allow the blastpipe to lift and expand under heavy working. Courtesy of '6023 King Edward II Project'.

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