British Rail Class 37 (English Electric Type 3)

The British Rail Class 37 is one of those British machines that solved a problem so thoroughly it refused to leave. Built as the English Electric Type 3, it was ordered to do a simple, very 1960s job: replace steam quickly with a modern diesel that could handle mixed-traffic duties—passenger, parcels, and freight—across a network that was still full of tight curves, short platforms, and weight-sensitive routes. It did that job, and then kept doing it for decades, turning up wherever a railway needed a capable, go-anywhere diesel with good adhesion, sensible power, and a reputation for getting on with it.

At a glance, the Class 37 is a Co-Co diesel-electric: six powered axles on two three-axle bogies, a layout chosen to spread weight and deliver traction on less-than-perfect rails. Under the skin sits the English Electric 12CSVT V12 engine, producing around 1,750 bhp in standard form. That output doesn’t sound huge next to later freight locomotives, but the 37 was designed for a Britain that still ran a wide spread of duties at moderate speeds, often on routes where axle load and route availability mattered as much as horsepower.

The result was a locomotive that became familiar almost everywhere—especially in East Anglia and Scotland, where 37s formed the backbone of important passenger services for years. They were also famously vocal: the type’s distinctive exhaust note earned it the nickname “tractor”, not as a compliment or an insult so much as a matter of acoustic fact. If you have heard a 37 open up under load, you do not need the explanation twice.

This page sets out what the class is and how it developed, without the soft-focus nostalgia. The Class 37’s story is an operational history: a fleet built in large numbers, adapted in mid-life, and then kept relevant through a mix of refurbishment, niche suitability, and the basic truth that railways don’t throw away a tool that still works.

Ordering, construction, and the “no prototype” approach

The Class 37 was ordered as part of British Rail’s wider modernisation drive. What is striking—particularly compared with some earlier diesel programmes—is that there was no single prototype built and tested before the main production run. British Rail and English Electric went straight to an order for multiple locomotives, reflecting both confidence in the design family and the urgency of replacing steam traction at scale.

Production ran from 1960 to 1965, with 309 locomotives built in total. Construction was split between Vulcan Foundry at Newton-le-Willows and Robert Stephenson and Hawthorns at Darlington, again reflecting the scale of the programme and the need to deliver quickly. Early locomotives carried “D” numbers (in two ranges), later renumbered under TOPS into the familiar 37xxx sequence.

Visually, the class sits firmly in the English Electric family. The body style is closely related to other contemporary EE designs, with variations appearing as the build continued—most notably changes around nose-end details and headcode arrangements. These are not merely cosmetic: they reflect the era’s obsession with headcodes, route identification, and the practical business of making locomotives easier to maintain and operate.

Technical outline: what makes a Class 37 a Class 37

Diesel-electric layout

The Class 37 is diesel-electric in the orthodox sense: the diesel engine drives a generator/alternator, producing electricity for traction motors, which in turn drive the axles. This arrangement suits a mixed-traffic locomotive because it provides controllable power at low speeds for freight and station work while still allowing sustained higher-speed running for passenger trains.

Co-Co running gear

A defining feature is the Co-Co wheel arrangement—six powered axles. This spreads weight and improves adhesion, which matters on Britain’s often damp, leaf-coated rails and on routes with lower axle load limits. For the operator, it also meant a reasonably capable locomotive that could go places heavier, more powerful types might not have been welcome.

Power and performance

Standard Class 37s are associated with the English Electric 12CSVT engine at roughly 1,750 bhp. Top speed is typically quoted around 90 mph for many versions, with some later gearing differences and operational limits depending on subclass and duty. The point is not that the 37 is a “fast” locomotive in a modern sense; it is that it was fast enough for the passenger work it was asked to do, and strong enough for the freight it was expected to haul—often without complaint.

Braking and multiple working

Over time, the class worked with vacuum braking, air braking, and dual configurations, reflecting the transition in British rolling stock practice. Multiple working was via the Blue Star system, which mattered in the days when double-heading was a practical answer to heavy trains and awkward gradients, and when rescue duties were an everyday reality rather than a special event.

Train heating: steam versus ETH

Like many British diesels designed in the overlap period between steam-heated coaching stock and electrically heated stock, Class 37s ended up with a complicated relationship with train heating. Some were fitted with steam heating equipment for passenger work in the early years. Later, as coaching stock standards changed, a portion of the class received Electric Train Heating (ETH) as part of refurbishment programmes—creating passenger-capable sub-fleets long after the locomotives were “meant” to have moved on.

Early service: mixed traffic in the real world

The 37 was conceived as a mixed-traffic type and was used accordingly. In the early years it worked:

• secondary passenger services on non-electrified main lines,
• parcels and mail trains where timing and reliability mattered,
• freight turns where its power and adhesion were a good match,
• and, inevitably, the odd task nobody quite planned for—because a locomotive that can do most things ends up doing nearly everything.

Two areas became particularly associated with the class.

East Anglia: InterCity by Type 3

For long stretches of the diesel era, East Anglia’s principal services were not the preserve of glamorous traction. Class 37s became a mainstay of InterCity and longer-distance passenger work there, hauling trains at respectable speeds on routes that demanded reliability and flexibility more than raw power.

Scotland: long distances, harsh weather, hard work

In Scotland, Class 37s became even more culturally and operationally embedded. They worked important passenger services, and later became heavily associated with rugged, long-distance duties where a sure-footed diesel with strong adhesion and proven engineering was valuable. Scotland’s geography and weather are effective at exposing weaknesses; the 37’s long service there tells you how it coped.

TOPS renumbering and the settling of identities

The introduction of TOPS brought the familiar Class 37 identity and the 37xxx numbering series. In principle, renumbering was sequential: a “D” number became a “37” number. In practice, as with many classes, there were exceptions and quirks driven by withdrawals, accidents, and the administrative realities of handling hundreds of locomotives across multiple regions.

For the working railway, TOPS mattered because it supported a modern approach to asset tracking, faults, maintenance history, and allocation. For the Class 37, it also marked a transition from a new diesel type to a mature fleet that the railway expected to keep around—and therefore needed to manage properly.

Mid-life refurbishment and the creation of subclasses

By the 1980s, the Class 37 was no longer new, but it was still useful. British Rail’s response was not simply to withdraw the class and replace it wholesale; it was to refurbish and reconfigure substantial parts of the fleet to suit changing needs. This is where the subclass story becomes central.

The key idea is simple: the “Class 37” label covers a family of locomotives that were modified in different ways for different roles. In broad terms:

• 37/0 refers to locomotives that remained broadly in original form after other sub-fleets were created.
• 37/3 generally refers to locomotives that were re-bogied (notably with CP7 bogies in some cases) but not fully refurbished.
• 37/4 became the most passenger-focused refurbishment: refurbished, rewired, and fitted with ETH, with alternator conversions and other changes intended to give a dependable passenger locomotive in the sectorisation era.
• 37/5 broadly refers to refurbished locomotives without ETH.
• 37/6 reflects further modifications for specific operational needs, including through wiring arrangements and jumper cables to work with certain stock.
• 37/7 describes heavily modified, ballasted locomotives intended for heavy freight, particularly where additional adhesion was needed.
• 37/9 refers to a small set of heavily modified testbed locomotives fitted with different engines (Mirrlees or Ruston) and altered electrical equipment, associated with experimental evaluation rather than large-scale fleet conversion.

These categories matter because they explain why a locomotive designed in the late 1950s could remain operationally relevant long after newer traction appeared. A railway that can rewire and re-equip a known platform can buy time—and, if the conversion is good enough, buy capability.

Class 37/4: refurbish, rewire, keep passenger diagrams alive

The 37/4 sub-fleet is the clearest example of mid-life adaptation. As ETH coaching stock became the standard, locomotives without ETH were pushed out of many passenger duties, regardless of how well they ran. The solution was to create a batch of ETH-fitted 37s through refurbishment at Crewe Works in the mid-1980s.

The work typically included extensive rewiring, replacement of certain generator arrangements with alternator equipment, and other changes aimed at improving reliability and suitability for passenger service. The refurbished locomotives emerged in a very visible era of British Rail branding, and they were used on passenger work in places where the 37 remained the right tool—especially in Scotland and Wales, where route characteristics and service patterns still suited locomotive-hauled operations.

The significance is not the paintwork or the renumbering. It is that British Rail was still willing to invest in 20-plus-year-old locomotives because the economics and operational needs made sense. A 37/4 was a practical answer to a practical problem: keep passenger services running with locomotives the railway already understood, rather than wait for an ideal replacement.

Class 37/7: ballast, adhesion, and the heavy-freight job

Where the 37/4 was about passenger capability, the 37/7 was about brute practicality: improving traction for heavy freight by adding ballast weight and altering gearing and equipment. Ballasting increased the locomotive’s weight significantly (commonly quoted to around 120 tons for the heavy variants), improving adhesion—useful for difficult freight flows, including heavy coal and other demanding duties where wheel-slip could be an operating headache.

This is the Class 37 logic pushed in a different direction: if you cannot justify a completely new locomotive class for a niche, but you have a solid platform and the skills to modify it, you modify it. The 37/7s were not built to be glamorous; they were built to make heavy trains move reliably in the real world.

Class 37/9: the “Slugs” and the experimental branch

The 37/9 locomotives are a small but notable corner of the story. In the late 1980s, a handful of Class 37s were rebuilt as testbeds for alternative engines and electrical equipment—Mirrlees in some, Ruston in others—linked to wider thinking about future freight traction and component evaluation. They were heavily modified, ballasted, and visually distinctive, with changes around grilles and body details.

For a fleet history, the important point is scale: this was not a mass conversion. It was experimentation using an available platform. It illustrates something else about British railway engineering culture: when you have a sturdy locomotive with space and structural strength, it becomes a convenient test mule. The Class 37 was, in that sense, useful even as an experimental chassis.

Sectorisation and late BR: the 37 finds its niches

By the late 1980s and early 1990s, British Rail’s sectorisation era placed sharper demands on traction. Locomotives needed to match roles: freight sectors wanted reliable haulage; passenger sectors wanted heating capability and timetable resilience; engineers wanted robust machines for infrastructure work.

The Class 37 responded by becoming more specialised in practice, even if the class as a whole remained “mixed-traffic” in heritage terms. ETH-fitted examples continued passenger work where required. Non-heated examples found freight and engineers’ roles. Modified heavy variants tackled difficult loads. And as newer types arrived, the 37 increasingly became the locomotive that filled the gaps—because it was available, understood, and adaptable.

After privatisation: small fleets, specialist work, and a long tail

Privatisation did not kill the Class 37; it changed the way it was used.

Large-scale, everyday passenger work diminished as multiple units expanded and newer traction took over. Freight operators introduced newer locomotives for mainstream flows. But the 37 retained a foothold because railways always have awkward corners: duties where a specific combination of route availability, traction characteristics, braking, and compatibility makes an older type still valuable.

In the post-privatisation era, Class 37s have been used for:

• infrastructure and engineering trains, including testing and support duties,
• freight flows where the locomotive’s characteristics fit the job,
• empty stock movements and occasional passenger substitution,
• charter and special trains, where locomotive-hauled stock remains in demand.

It is also common for locomotives to move between ownership structures: preserved examples returning to main-line use, locomotives sold to leasing and spot-hire companies, and machines exported for overseas engineering projects. The 37’s long working life has produced a complex ownership map, but the underlying reason for its survival is consistent: it can still do useful work.

The modern period: still in service, but the retirement notices are real

By the mid-2020s, the Class 37 was well beyond what most industries would consider a normal asset life, yet “numerous examples” remained in main-line service in Britain. Operators with small fleets have used them where they make sense—often in specialist or heritage-aligned roles rather than as day-to-day fleet backbone.

At the same time, modern pressures are obvious and practical. Emissions expectations are rising. Parts supply is becoming more specialised. Safety and operational systems continue to evolve. Fleet strategies increasingly favour standardisation around newer types.

Some operators have publicly signalled the end of Class 37 use as part of modernisation and environmental commitments. That does not mean the class vanishes overnight. It means the story tilts from “how are they still going?” to “where will the last few niches remain?”

Preservation: common, capable, and genuinely useful

The Class 37 is heavily represented in preservation, partly because it is a popular type and partly because it is a practical one. Heritage railways need locomotives that can:

• move trains reliably at modest speeds,
• operate economically compared with large main-line types,
• and handle demonstration freight, station pilot work, or service trains.

A Class 37 fits that bill, and many preserved examples are operational rather than static. Preservation is not the point of the class’s existence, but it is now part of the class’s factual footprint: many locomotives survive, and a portion of them continue to run.

Summary: what the Class 37 is, in one paragraph

The British Rail Class 37 (English Electric Type 3) is a 1960–1965 built, 309-strong class of Co-Co diesel-electric locomotives powered in standard form by the English Electric 12CSVT engine at around 1,750 bhp. Designed for mixed-traffic duties, it became particularly associated with passenger services in East Anglia and Scotland, while also working freight, parcels, and engineers’ trains across the network. In mid-life the fleet was extensively modified into multiple subclasses—most notably the ETH-fitted 37/4 for passenger work and the ballasted 37/7 for heavy freight—helping the class remain operationally relevant long after newer traction arrived. In the post-privatisation era, smaller fleets continued in specialist roles, and the type became widely preserved, ensuring that one of Britain’s most recognisable diesel designs remains part of the railway landscape well into its seventh decade.