GWR 4073 Class

GWR 4073 Castle class

5034 Corfe Castle fresh from Swindon Works, 1954.

Type and origin Power type Steam Designer Charles Collett Builder GWR / BR Swindon Works Build date 1923–1950 Total produced 171

Specifications Configuration 4-6-0 UIC class 2'Ch4 Gauge 4 ft 8 1⁄2 in (1,435 mm) standard gauge Leading dia. 3 ft 2 in (0.965 m) Driver dia. 6 ft 8 1⁄2 in (2.045 m) Minimum curve 8 chains (530 ft; 160 m) normal, 7 chains (460 ft; 140 m) slow Length 65 ft 2 in (19.86 m) over buffers Width 8 ft 11 in (2.718 m) Height 13 ft 4 1⁄2 in (4.077 m) Axle load 19 long tons 14 cwt (44,100 lb or 20 t) 22.1 short tons full Adhesive weight 58 long tons 17 cwt (131,800 lb or 59.8 t) 65.9 short tons full Loco weight 79 long tons 17 cwt (178,900 lb or 81.1 t) 89.4 short tons full Tender weight 47 long tons 6 cwt (106,000 lb or 48.1 t) 53.0 short tons full Fuel type Coal Fuel capacity 6 long tons 0 cwt (13,400 lb or 6.1 t) 6 long tons 0 hundredweight (6.10 t; 6.72 short tons) Water cap 3,500 imp gal (16,000 l; 4,200 US gal) - 4,000 imp gal (18,000 l; 4,800 US gal) Firebox:  • Firegrate area 29.36 sq ft (2.728 m2) Boiler pressure 225 lbf/in2 (1.55 MPa) Heating surface:  • Tubes 1,857.7 sq ft (172.59 m2) (Collett) 1,799.5 sq ft (167.18 m2) (Hawksworth)  • Firebox 162.7 sq ft (15.12 m2) (Collett) 163.5 sq ft (15.19 m2) (Hawksworth) Superheater: ​  • Type 14-element "Swindon" (Collett) 21-element (Hawksworth)  • Heating area 262.6 sq ft (24.40 m2) (Collett) 295.0 sq ft (27.41 m2) (Hawksworth) Cylinders Four (two inside, two outside) Cylinder size 16 in × 26 in (406 mm × 660 mm) Valve gear Inside cylinders: Walschaerts Outside cylinders: derived from inside cylinders via rocking bars. Valve type Piston valves

Performance figures Tractive effort 31,625 lbf (140.68 kN)

Career Operators Great Western Railway British Railways Power class GWR: D BR: 7P Numbers 4073–4099; 5000–5099; 7000–7037. Axle load class GWR: Red Withdrawn May 1950 to December 1965 Disposition Eight preserved, remainder scrapped

The 4073 Class or Castle class were 4-6-0 steam locomotives of the Great Western Railway design built between 1923 and 1950. They were designed by the railway's Chief Mechanical Engineer, Charles Collett, for working the company's express passenger trains.

The origins of this highly successful design date back to the Star Class of 1907 which introduced the basic 4-cylinder 4-6-0 layout with long-travel valves and Belpaire firebox that was to become synonymous with GWR express passenger locomotives. The Star class were designed to take the top express trains on the GWR with 61 in service by 1914, but after World War 1 there was a need for an improved design. To meet this need Chief Mechanical Engineer GJ Churchward had in mind an enlarged Star class design with a standard No.7 boiler, as fitted to his GWR 4700 Class express freight 2-8-0 design. However, this combination would have taken the axle load of such a design over the 20 ton limit then set by the civil engineers, and in the end nothing came of the idea.

C. B. Collett succeeded Churchward as Chief Mechanical Engineer of the GWR in 1922 and immediately set about meeting the needs for a new locomotive design that would both supplement the Stars and replace them on the heaviest expresses. Collett's solution was to take the basic layout of the Star with an extended frame and add a newly designed No.8 boiler which was both larger and lighter. The increased amount of steam that this produced allowing an increase in the cylinder diameter from 15 in × 26 in (381 mm × 660 mm) to 16 in × 26 in (406 mm × 660 mm). The extended frame allowed for a side window cab and an increased grate area. The result was an increase in tractive effort to 31,625 lb, and a locomotive that looked attractive and well proportioned while remaining within the 20 ton axle limit.

Unlike the Star class, there was no prototype. Collett was sufficiently confident of the design to place an order with Swindon railway works (Lot 224) for ten locomotives in 1923 although there was a four-month delay between the appearance of the first example, in August 1923 and the second in the December to allow for the correction of any teething problems. Thereafter the remaining eight locomotives came out at regular intervals until April 1924. They were 4073-4082; the number series continuing unbroken from the Star class. The last 12 Star class locomotives, which were built in 1922-23, had been given names of Abbeys in the western area served by the GWR. The new locomotives were named after castles also in the west beginning with 'Caerphilly Castle'. Over the twenty-seven years from August 1923 to August 1950 155 Castles were built new at Swindon Works and a further sixteen were converted from other classes. In February 1952, two engines, 4082 Windsor Castle and 7013 Bristol Castle swapped names and numbers with each other because King George VI died. 7013 was disguised as 4082 to run the funeral train and the two engines never swapped back. 4082 was withdrawn from service in 1964 as 7013 and 7013 was withdrawn from service as 4082 in 1965.