LNWR Jubilee Class

The first two of this class were prototypes, built to different designs to permit a comparison. The first, No. 1501 the Iron Duke (later re-named Diamond Jubilee and then Jubilee), was a 4-cylinder simple locomotive with 15 in × 24 in (381 mm × 610 mm) cylinders. The second, No. 1502 Black Prince, was not Webb's first compound, but was his first 4-cylinder compound and the first in the UK.^[1] The outside high-pressure cylinders were the same as Iron Duke's, the inside low-pressure cylinders were 19+1⁄2 in × 24 in (495 mm × 610 mm).^[2]

There was no simpling valve or other means for starting, and so when starting they just operated as small 2-cylinder simples.^[1] The Joy valve gear was shared between high and low pressure, with a rocking lever to the high-pressure valves. The inside cylinders were angled above the outside cylinders and although this could have been solved by cranking the rocking levers, this gave an uneven drive to the valves; valve-setting between both of them had to be a compromise position, ideal for neither, and so gave an uneven power distribution between high and low. Their running was thus not as free-running as it might have been, which Webb would address in his later designs.^[1]

These were also Webb's first designs with a leading bogie rather than a pony truck,^[2] which was also described as a "double radial truck".^[1][3] The truck pivots geometrically at a point behind its rear axle, although there is no single mechanical pivot point. This motion is controlled instead, like Webb's earlier single radial truck, by curved radial slides, with a radius of curvature at the centre of the truck of 10 ft 9 in (3.28 m).^[4]

Another novel feature was that both locomotives were fitted with double chimneys. The smokebox was partitioned internally into upper and lower halves, the lower section exhausting through the front chimney and the upper tubes through the rear chimney. The blastpipes were fed separately, the front chimney from the left cylinders and the rear from the right.^[2] After some time in service, the two chimneys were replaced with a single chimney on an undivided smokebox and their performance and fuel consumption measured again. It was found that the compound locomotive had identical performance both with and without the double chimney, but that the simple locomotive was improved by it.^[2] The double chimney was re-fitted to Jubilee, but the production locomotives were built as compounds without it.

After around a year's running, the simple had run for 33,517 miles, with an average coal consumption of 40.3 pounds per mile and the compound (starting slightly later) for 23,503 miles with a consumption of 38.1 pounds per mile. The compound was thus cheaper by 2.2 pounds per mile, or 5%.^[2]

The ratio between LP and HP cylinders was 1.69, lower than that considered optimal.^[3] Webb's 3-cylinder compounds had used the more usual figure of 2. To improve this to 1.87, Webb had decided to increase the size of the LP cylinders to 20+1⁄2 in × 24 in (521 mm × 610 mm), and this was applied to the production locomotives.^[2]