British Rail Class 21 (NBL)

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North British Type 2 diesel-electric
British Railways Class 21
Harringay West geograph-2392160-by-Ben-Brooksbank.jpg
D6103 at Harringay West in 1959.
Type and origin
Power typeDiesel-electric
BuilderNorth British Locomotive Co.
Build date1958–1960
Total produced58
 • UICBo'Bo'
 • CommonwealthBo-Bo
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Wheel diameter3 ft 7 in (1.092 m)
Minimum curve3.5 chains (231.00 ft; 70.41 m)
Wheelbase37 ft 0 in (11.28 m)
Length51 ft 6 in (15.70 m)
Width8 ft 8 in (2.64 m)
Height12 ft 8 in (3.86 m)
Loco weight72.50 long tons (73.7 t; 81.2 short tons)
Fuel capacity460 imp gal (2,100 l; 550 US gal)
Prime moverMAN L12V18/21S
Traction motorsGEC WT440, 4 off
TransmissionDiesel electric
MU workingD6100–D6137: Red circle
D6138–D6157: Blue star
Train heatingSpanner 1,500 pounds (680 kg) per hour Steam generator
Train brakesVacuum
Performance figures
Maximum speed75 mph (121 km/h)
Power outputEngine: 1,000 hp (746 kW) or 1,100 hp (820 kW)
Tractive effortMaximum: 45,000 lbf (200.2 kN)
Brakeforce50 long tons-force (500 kN)
OperatorsBritish Railways
Axle load classRoute availability 6 (5 from 1969)
Disposition20 rebuilt to Class 29, remainder scrapped

The British Rail Class 21 was a type of Type 2 diesel-electric locomotive built by the North British Locomotive Company in Glasgow for British Railways in 1958-1960. They were numbered D6100-D6157.[1]


Under the British Railways Modernisation Plan, a batch of ten 1,000 hp (746 kW) diesel-electric locomotives were ordered from the North British Locomotive Co. for evaluation under BR's dieselisation pilot scheme. At the same time, six externally similar locomotives employing hydraulic transmission were ordered for comparison, these becoming Class 22. Repeat orders resulted in a total of 58 of the diesel-electric locomotives being built (numbered D6100–6157). They were delivered between December 1958 and November 1960.


Eastern Region[edit]

The first 38 locomotives entered service in 1958-59 from the Eastern Region depots at Stratford, Hornsey and Ipswich engine shed on commuter services into London, where they were evaluated against rival designs from English Electric, British Railways, Birmingham RC&W and Brush. The type proved chronically unreliable in Eastern Region service - by March 1960 the Hornsey allocation had moved to New England Yard, Peterborough for storage. This came to the attention of the newspapers and the Daily Telegraph reported that brand new diesel locomotives were being hidden and dumped, which caused questions to be asked in Parliament.[2] The D6100s moved north to the Scottish Region in mid-April 1960, ostensibly to be nearer to the NBL works for repairs but allegedly to move them away from the eyes of the national press.[3]

Scottish Region[edit]

The final 20 locomotives had uprated 1,100 hp (820 kW) engines and were delivered to Kittybrewster depot on the Scottish Region. They were joined on the Scottish Region by the first 38 locos, which were allocated to Glasgow Eastfield depot, close to the North British factory at Springburn where they had been built. They were used widely across the Scottish Region on a range of work, freight, local passenger and express passenger, the latter sometimes in pairs. Common double headers included Oban & Callander workings, Glasgow-Dundee/Aberdeen expresses, and many freights, and the Ballater Royal train was entrusted to two locos with a standby.[citation needed] They were common on the West Highland lines, Great North of Scotland lines and in the works.[clarification needed] Just one original locomotive, D6109, was repainted in BR Blue with headcodes.


They proved to be unreliable in service, and during 1960 the Eastern Region fleet was transferred to Eastfield depot on the Scottish Region for convenience of return to their manufacturer when warranty work was required. However, the North British Locomotive Works closed in 1962, by which time the type's principal shortcomings had become plain. D6100s suffered problems with the coupling between the power unit and the generator. The engines themselves were a MAN design, but which were built under licence by NBL and of inferior quality to the German originals.[4] Engine cooling systems proved to be inadequate, diesel engines leaked and were not constructed to the appropriate tolerances, cylinder heads fractured and lubricating oil escaped into the battery compartments located below the power unit. These flaws were mostly rectified on a rebuilding programme in 1961-62. The positioning of minor components within the locomotive bodyshell meant that small faults could only be rectified on depot or by return to a railway workshop, which resulted in poor daily availability for traffic figures for the type. Engine room fires were common and wrote off several locomotives.[5] However the later batch, with the B series engine, proved more reliable but still suffered problems. Problems with the NBL/MAN engines are detailed in a report prepared at Swindon in September 1962. This related to the Class 22 but similar problems were experienced with the Class 21.[6]


In an attempt to improve reliability 20 locomotives, (D6100–03, D6106-D6108, D6112–D6114, D6116, D6119, D6121, D6123, D6124, D6129, D6130, D6133, D6134 and D6137), were re-engined between 1963 and 1965 with 1,350 hp (1,010 kW) Paxman Ventura engines to form Class 29.


The remaining 38 locomotives retained their original NBL/MAN engines until they were withdrawn from service between December 1967 and August 1968 and sold for scrap. Most were cut up by Scottish scrap dealers McWilliams of Shettleston or Barnes and Bell of Coatbridge, but locomotive D6122 was sold to Woodham Brothers scrapyard in Barry, Vale of Glamorgan, South Wales, where it languished until 1980 before being broken up. None have survived.


  1. ^
  2. ^ Clough, David (2005). Diesel Pioneers. Ian Allan. p. 65. ISBN 0 7110 3067 7.
  3. ^ Clough, David (2005). Diesel Pioneers. Ian Allan. p. 67. ISBN 0 7110 3067 7.
  4. ^ Haresnape 1983, p. 52.
  5. ^ Clough, David (2005). Diesel Pioneers. Ian Allan. p. 64. ISBN 0 7110 3067 7.
  6. ^


  • Ian Allan ABC of British Railways Locomotives, summer 1966 edition
  • Grindlay, Jim. British Railways Locomotive Allocations 1948-1968 (Part 6 - Diesel & Electric Locomotives). Troon: Modelmaster Publications. ISBN 978-0-9544264-6-0.
  • Haresnape, Brian (February 1983). 4: Production Diesel-Electrics Types 1-3. British Rail Fleet Survey. Shepperton: Ian Allan. ISBN 0-7110-1275-X. GE/0283.
  • Stevens-Stratten, S.W.; Carter, R.S. (1978). British Rail Main-Line Diesels. Shepperton: Ian Allan Ltd. ISBN 0-7110-0617-2.
  • Sugden, S.A. Diesel & Electric Loco Register (3rd edn). Sheffield: Platform 5. ISBN 1-872524-55-9.