British Rail Class 37

The Class 37's origins lie in BR's 1960 motive power requirements, which called for a Type 3 (1,750 hp) mixed-traffic locomotive to complement the smaller Type 1s and Type 2s of the BR Pilot Scheme era. English Electric proposed a Co-Co design using a twelve-cylinder Vee-form development of the 8SVT engine used in the Class 20. The 12CSVT was a turbocharged engine of 1,750 hp, sharing many components with the 8SVT and 16SVT (Class 40) but offering the Type 3 power output in a lighter and more compact form than a sixteen-cylinder engine.

The Co-Co bogie was selected over the alternative Bo-Bo to give better adhesion for heavy freight work and broader route availability through lower individual axle loads. The body styling followed English Electric's then-current Vulcan Foundry house style — a clean twin-cab body with the engine compartment between the cabs. The first locomotive, D6700, was completed at Vulcan Foundry in December 1960; production continued at Vulcan and at RSH Newcastle through to 1965 in batches of around 50 per year, by which time 309 had been built.

The basic design proved exceptionally robust and the 12CSVT engine reliable. From the late 1970s onwards a series of refurbishment and rebuild programmes produced the various sub-classes (37/3, 37/4, 37/5, 37/6, 37/7, 37/9) that prolonged the class's active life by several decades.

The Class 37 was the standard British Type 3 mixed-traffic diesel from 1960 onwards. It worked freight, secondary passenger, and (in upgraded sub-classes) main-line passenger work across virtually the entire BR system. The class's reputation was made by its versatility and reliability: 37s could be found on Cornish china clay one day and Scottish sleepers the next.

The Class 37/4 sub-class of 31 ETH-fitted locomotives, rebuilt at Crewe Works in 1985–1986, gave the class a notable second life as the Scottish Region long-distance passenger fleet, working the West Highland line (Glasgow–Mallaig and Glasgow–Oban), the Far North line (Inverness–Wick), and the Kyle line (Inverness–Kyle of Lochalsh) for many years. The 37/4 "Highland" examples are particularly fondly remembered by enthusiasts.

Withdrawals began on a significant scale from the early 1990s as freight traffic declined and the Class 60 and subsequently the Class 66 took over the heavier freight work. However, a substantial fleet passed to the post-1996 freight operators and to Direct Rail Services, where the 37 became a standard infrastructure and nuclear-flask locomotive. In 2026 around forty Class 37s remain in active mainline service, sixty-six years after the first was built — among the longest-lived British diesel classes ever produced.

Distinctive Co-Co diesel-electric with twin cab body and characteristic English Electric front-end styling. Original Phase 1 examples had split headcode panels above the cab; later Phase 2 examples had revised flat-front cab with the headcode box recessed. Sub-class identifying features: 37/0 original equipment; 37/3 with CP7 bogies and revised silencers; 37/4 with ETH (electric train heating) generator visible behind the cab — these were the principal Scottish Region passenger sub-class; 37/5 with refurbished freight-spec equipment; 37/6 with extra equipment for the abortive Eurostar Nightstar services; 37/7 with extra ballast for heavy freight work; 37/9 with non-EE engines (Mirrlees or Ruston-Paxman). The deep growling engine note when accelerating is characteristic.

D6700 (later 37001 then 37350), the first of the class, completed at Vulcan Foundry in December 1960. Withdrawn 1999 and preserved by the National Collection at the National Railway Museum.

37025 Inverness TMD, a long-serving Scottish Region 37/0 preserved at the Bo'ness and Kinneil Railway. The named-plate locomotive carries the name of the Scottish depot it served from for many years.

37109, an early 37/0 preserved at the South Devon Railway.

37411 The Scottish Railway Preservation Society, a 37/4 with ETH preserved at Bo'ness.

37706, 37710, 37716, etc. — heavy-freight 37/7 sub-class examples, some of which remained in mainline service into the 2020s.

37901 Mirrlees Pioneer, the leading Class 37/9 with Mirrlees MB275T engine, a one-off test bed. Preserved.

The active Class 37 fleet in 2026 includes examples with DRS, Colas Rail, GBRf, West Coast Railways, and Rail Operations Group; the fleet is fluid and current detail should be consulted from operator pages.

BR (1960–1996): the class was distributed across the BR system from 1960 onwards. Major allocations included Stratford (Eastern Region freight and passenger), Tinsley (Yorkshire freight), Healey Mills (Trans-Pennine freight), Cardiff Canton (South Wales), Plymouth Laira (Cornish china clay), Eastfield and Inverness (Scottish Region), and Crewe (West Coast freight). The Scottish Region passenger fleet (37/4 sub-class) was concentrated at Eastfield (Glasgow) and Inverness for the West Highland and Far North lines. The East Suffolk line saw 37s as standard freight and passenger motive power. Cornish china clay traffic was a 37 stronghold for many years. The class also worked the Cambrian Coast, North Wales coast, Settle and Carlisle, and North Country freight networks.

Privatisation onwards (1996–present): EWS (subsequently DB Cargo), Direct Rail Services, Colas Rail, GB Railfreight, West Coast Railways, and Rail Operations Group all operated Class 37s. DRS in particular based a substantial fleet at Carlisle Kingmoor and Crewe Gresty Bridge for nuclear flask trains, infrastructure work, and charter passenger services. In 2026 around forty Class 37s remain in mainline service.

BR Brunswick green (1960–1968): the class was outshopped in BR Brunswick green with a small lower-bodyside warning panel.

BR Rail blue (1965–1987): from the mid-1960s the class progressively wore standard BR Rail blue with full yellow ends.

Large logo blue (1980s): a notable variant carried by some 37/4 Scottish Region examples and others, with an enlarged BR "double arrow" logo on the bodyside.

BR sectorisation (1987–1996): Trainload Construction (light grey), Railfreight Coal (Triple-grey), Railfreight Petroleum, Railfreight Distribution, Civil Engineers (yellow/grey "Dutch"), and Mainline Freight (aircraft blue) were all carried by the class. The 37/4 Scottish passenger sub-class wore InterCity Mainline and ScotRail liveries in the late 1980s.

EWS, DB, DRS, Colas, GBRf, WCR, ROG (1996–present): the class has carried EWS maroon, DB red, DRS Compass blue, Colas yellow/orange, GBRf orange/blue, West Coast Railways maroon, and ROG dark blue. Many preserved examples have reverted to their original BR Brunswick green or BR Rail blue.