| North British Type 2 diesel-electric British Railways Class 21 |
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| D6103 at Harringay West in 1959. | |
| Power type | Diesel-electric |
| Builder | North British Locomotive Co. |
| Build date | 1958–1960 |
| Total produced | 58 |
| Configuration | Bo-Bo |
| UIC classification | Bo'Bo' |
| Gauge | 4 ft 8 1⁄2 in (1,435 mm) Standard gauge |
| Wheel diameter | 3 ft 7 in (1.092 m) |
| Minimum curve | 3.5 chains (70 m) |
| Wheelbase | 37 ft 0 in (11.28 m) |
| Length | 51 ft 6 in (15.70 m) |
| Width | 8 ft 8 in (2.64 m) |
| Height | 12 ft 8 in (3.86 m) |
| Locomotive weight | 72.50 long tons (73.7 t) |
| Fuel capacity | 460 imp gal (2,100 l; 550 US gal) |
| Prime mover | MAN L12V18/21S |
| Traction motors | GEC WT440, 4 off |
| Transmission | DC generator, DC traction motors |
| Multiple working | D6100–D6137: ● Red circle D6138–D6157: ★ Blue star |
| Top speed | 75 mph (121 km/h) |
| Power output | Engine: 1,000 hp (746 kW) or 1,100 hp (820 kW) |
| Tractive effort | Maximum: 45,000 lbf (200.2 kN) |
| Train heating | Spanner 1,500 pounds (680 kg) per hour Steam generator |
| Locomotive brakeforce |
50 LTf (500 kN) |
| Train brakes | Vacuum |
| Career | British Railways |
| Number | D6100–D6157 |
| Axle load class | Route availability 6 (5 from 1969) |
| Retired | 1967–1968 |
| Disposition | 20 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.
Since 2007, the Class 21 TOPS classification has been reused for new Vossloh G1206 locomotives acquired by Euro Cargo Rail/EWS.
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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.
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. They were only used in this area for one or two years before all being transferred to the Scottish Region at Glasgow Eastfield.
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.
They proved to be unreliable in service, and by the end of 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. In particular there were 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.[1]
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 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.
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