GWR 2900 Class - Prototypes

Prototypes

Three prototype two-cylinder 4-6-0 locomotives were constructed by the GWR in 1902 and 1903. The first, no. 100, was completed in March 1902. It had a domeless parallel boiler, raised Belpaire firebox, 19 in (48.3 cm) diameter outside cylinders with 30 in (76.2 cm) piston stroke, and boiler pressure of 200 psi (1.38 MPa). The piston valves were driven by rocking levers actuated by the expansion link of Stephenson valve gear- this particular design was only used on no. 100. The parallel boiler was later replaced with a half-cone boiler, and then the first superheated half-cone boiler in 1910. G J Churchward had studied American boiler design, but he was also influenced by continental practice in efficient motion design. From the Société Alsacienne de Constructions Mécaniques (SACM) a De Glehn 4-4-2 compound engine was ordered for comparative trials on the GWR.

Number 98, the second prototype locomotive, was built with a half-cone boiler and a re-designed valve gear layout and cylinders. Valve diameters were increased from 6+1⁄₂ inches (170 mm) to 10 inches (250 mm).

Locomotive no. 171, the third prototype, was built as a 4-6-0 but was soon converted to 4-4-2 for the De Glehn trials. Boiler pressure was increased to 225 psi (1.55 MPa).

Even whilst 171 was undergoing trials nineteen locomotives were ordered to be built to a similar design, of which thirteen were built as 4-4-2s and six as 4-6-0s.