Until the mid-1920s, the LMS had followed the Midland Railway’s small engine policy, which meant that it had no locomotives of sufficient power for its expresses on the West Coast Main Line. These trains were entrusted to pairs of LMS/MR Midland Compound 4-4-0s between Glasgow and Carnforth, and a 4-6-0 locomotive of the LNWR Claughton Class, piloted by an LNWR George V 4-4-0, southwards to Euston station.

The Operating and Motive Power Departments of the LMS were satisfied with the small engine policy. However, in 1926 the Chief Mechanical Engineer, Henry Fowler, began the design of a compound Pacific express locomotive. The management of the LMS, faced with disagreement between the CME and the other departments, obtained a loan of a GWR Castle class locomotive, Launceston Castle, which was operated for one month between Euston and Carlisle.

Following the success of the Castle 4-6-0 in working on the LMS, a decision was taken to cancel Fowler’s Pacific project, and to replace it with a 4-6-0 with three cylinders and a simple-expansion steam circuit. Because there was an urgent need for new express locomotives the LMS placed an order with the North British Locomotive Company of Glasgow for 50 engines. The North British, with its extensive drawing office and two works, possessed sufficient capacity to expedite the order within a year. The Derby drawing office and North British staff collaborated in designing the class, with the latter producing the working drawings. Fowler took little part in the design process, which was carried out by Herbert Chambers, Chief Draughtsman at Derby, and his staff. The LMS requested a set of drawings of the Castle class from the GWR, but didn’t receive them. Instead a set of drawings of the SR Lord Nelson Class were obtained, and used for the design of the firebox. The main features of the design followed existing Derby practice, with the cylinders and valve gear being derived from the Fowler 2-6-4T, also being designed at Derby at that time.

They were introduced without testing. Radford claims that the boiler owed much to the MR 0-10-0 Lickey Banker ‘Big Bertha’. A further 20 were built by Derby Works.

They were initially named after regiments of the British Army, and after historical LNWR locomotives. Those with LNWR names were renamed in 1935 and 1936 with more names of regiments.

Type and origin
Power typeSteam
DesignerSir Henry Fowler
rebuilt: Sir William Stanier
  • North British Locomotive Company (50)
  • LMS, Derby Works (20)
Order numberLMS Lot 41 (50) and 73 (20)
Serial numberNBL: 23595–23644
Build date1927, 1930
Total produced70
RebuilderLMS Derby Works
Rebuild date1943–1955
 • Whyte4-6-0
 • UIC2′C h3
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Leading Wheels dia.3 ft 3 12 in (1.003 m)
Driving Wheels dia.6 ft 9 in (2.057 m)
Loco weight84.90 long tons (86.26 t; 95.09 short tons)
Tender weightNew: 42.70 long tons (43.39 t; 47.82 short tons)
later: 54.65 long tons (55.53 t; 61.21 short tons)
Water capNew: 3,500 imp gal (16,000 l; 4,200 US gal)
later: 4,000 imp gal (18,000 l; 4,800 US gal)
Tender cap.New: 5.5 long tons (5.6 t; 6.2 short tons)
later: 9 long tons (9.1 t; 10 short tons)
BoilerG10¼S; rebuilt: 2A
Boiler pressure250 psi (1.72 MPa) superheated
Cylinder size18 in × 26 in (457 mm × 660 mm)
Valve gearWalschaerts
Valve typePiston valves
Performance figures
Tractive effort33,150 lbf (147.46 kN)
  • London, Midland and Scottish Railway
  • → British Railways
Power class6P; reclassified 7P in 1951
  • LMS: 6100–6169
  • BR: 46100–46169
Axle load classBR: Route Availability 9
DispositionTwo preserved; remainder scrapped

More information available at Wikipedia