High Pressure Experiment

During the 1920s much attention was paid to the possibility of using high steam pressure for locomotives, and H. N. Gresley decided to produce a high-pressure boiler suitable for his new Pacifics.

Working in conjunction with the firm of Yarrow, Gresley designed a water-tube boiler combining a pressure of 450 Ib per sq inch with an output equal to that of his standard Pacific boiler, and in 1929 the new locomotive first went on trial.

With a wheel arrangement of 4-6-4, No. 1000 was a 4-cylinder compound with two 10-in. and two 20-in. cylinders, driving wheels 6 ft. 8 in. in diameter, and a tractive effort of 32,000 Ib.

There were many other unusual features, such as telemotor control of the reversing gear, but in practice the high-pressure three-drum boiler provided most of the headaches found with any unorthodox design.

Various modifications were made; an intermediate superheater was fitted to avoid condensation in the low-pressure cylinders, and a starting regulator to allow working as a 4-cylinder simple.

Despite these changes, however, No. 10000 was heavy on coal and maintenance, and in 1937 was rebuilt as a 3-cylinder simple with a conventional 250-lb. boiler and a streamlined casing similar to the 'A4s.' Renumbered to 60700 after nationalization, this unique 4-6-4 continued in main-line passenger service until 1959, and is here shown leaving York with an express for London.