- Prime mover: Mirrlees MB275T (test‑bed conversion)
- The 37/9s were created to trial alternative engines (Mirrlees or Ruston‑Paxman)
- Role: Heavy freight, engineering trains, later storage, then mainline return (ROG / RailAdventure)
- Liveries: Mirrlees Pioneer, Rail Operations Group, Europhoenix
- Why it matters: 37901 is one of the rare experimental Class 37s, mechanically unique and historically important as a test locomotive.
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🟦 Side‑by‑side comparison
| Feature | 37423 | 37901 |
|--------|-------------|-------------|
| Sub‑class | 37/4 | 37/9 |
| Engine | Standard EE 12CSVT | Mirrlees MB275T experimental |
| Power role | Passenger (ETH‑fitted) | Freight / test‑bed |
| Major rebuild | 2008 modernisation (lights, cab, MU sockets) | Early‑1990s re‑engining |
| Typical operators | DRS, Europhoenix | ROG, RailAdventure, Europhoenix |
| Distinctive feature | Full ETH + modern lighting package | Unique Mirrlees engine & test‑loco status |
| Operational character | Smooth, reliable passenger tractor | Loud, unusual, mechanically unique |
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🟩 Why they feel so different in service
- 37423 sounds and behaves like a “classic” EE tractor — same 12CSVT growl, but with modernised cab and lighting.
- 37901 has a completely different engine note, different throttle response, and historically suffered reliability issues due to the experimental Mirrlees power unit.
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🟨 If you want deeper detail
I can map either locomotive’s full life story, including allocations, liveries, rebuild dates, and current status:
- Full history of 37423
- Full history of 37901
- Technical comparison of EE 12CSVT vs Mirrlees MB275T
Which one do you want to dive into next?
The core difference is this: the English Electric 12CSVT is a proven, reliable, 1,750 hp V12 turbo‑diesel that powered almost every Class 37, while the Mirrlees MB275T is a much newer, experimental straight‑six designed to produce 2,000+ hp, fitted only to the Class 37/9 test fleet — and it caused major reliability problems.
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🔧 1. Engine architecture
- 12CSVT: V12, turbocharged, 1,750 hp.
- Classic English Electric design, evolved from the 8SVT.
- Smooth, balanced, mechanically conservative.
- Known for reliability and long service life.
- Standard across Class 37/0, 37/4, 37/5, 37/6, 37/7.
- MB275T: Straight‑six, turbocharged, designed for 2,000+ hp.
- Much larger cylinders, fewer moving parts, theoretically lower maintenance.
- Taller and longer than the EE engine — required a redesigned “peaked” roof on 37/9s.
- Installed only in 37901–37904.
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⚙️ 2. Power output & performance
- 12CSVT:
- 1,750 hp gross.
- Optimised for mixed‑traffic work.
- Excellent throttle response and traction motor compatibility.
- Very stable at low speeds and under heavy load.
- MB275T:
- Capable of 2,000–2,250 hp, but derated to 1,800 hp in Class 37/9s to avoid overstressing EE traction motors.
- More torque and stronger mid‑range pull.
- Promised better fuel efficiency.
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🛠️ 3. Reliability & maintenance
- 12CSVT:
- Extremely reliable; one of BR’s most successful diesel engines.
- Crews liked its predictability and robustness.
- Minimal catastrophic failures across decades of service.
- MB275T:
- Suffered from persistent reliability issues during trials.
- Cardiff Canton struggled to keep the small fleet available.
- The experimental programme was ultimately abandoned.
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🔊 4. Sound & driving characteristics
- 12CSVT:
- Classic “tractor/growler” sound — deep, throaty, rising to a hard-edged roar.
- Smooth power delivery due to V12 configuration.
- MB275T:
- Completely different note — harsher, more industrial, with a distinct turbo whine.
- Heavier reciprocating mass made it feel more “agricultural” under throttle (inferred from mechanical design; not directly stated in sources).
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🧩 5. Operational context
- 12CSVT:
- Standard engine for the entire Class 37 fleet.
- Used for freight, passenger, test trains, RHTT, nuclear flask trains — everything.
- Still in service today.
- MB275T:
- Part of the aborted “Class 38” next‑generation diesel project.
- Only six locomotives re‑engined (four Mirrlees, two Ruston).
- Intended to test future BR diesel concepts.
- Withdrawn from the experiment after reliability problems.
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📊 Comparison table
| Feature | 12CSVT | Mirrlees MB275T |
|--------|-------------|----------------------|
| Cylinders | V12 | Straight‑six |
| Power | 1,750 hp | 1,800 hp (derated), capable of 2,000+ |
| Era | 1960s | 1986–87 experimental |
| Reliability | Very high | Problematic |
| Use | Standard Class 37 fleet | Only 37901–37904 |
| Roof profile | Standard | Peaked roof (engine height) |
| Purpose | Mixed‑traffic | Test‑bed for future BR diesel design |
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If you want, I can also compare the MB275T vs Ruston RK270T (the other 37/9 engine) or map the exact mechanical changes inside a 37/9.
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