HS4000 ‘Kestrel’ was a one-off prototype locomotive built by Brush Traction of Loughborough, England in 1967. It was designed as a technological showcase to attract buyers from both British Rail and overseas, and with a power output of 4000hp it remains the most powerful diesel locomotiveever to run in service on British railways, and one of the fastest with a top speed of 125mph, predating the IC125 HST’s by around a decade.
The name was derived from a combination of the initials of Hawker-Siddeley, Brush’s parent company, and that headline power output of 4000hp. Most, if not all, of the locomotives that were built at Loughborough carried the Brush name; Hawker-Siddeley was at this time synonymous with the manufacture of aircraft such as the ground-breaking (and highly successful and world-renowned) Harrier Jump-Jet, and was likely chosen for its higher profile and more glamorous associations, particularly where foreign buyers were concerned.
The prime mover was a turbocharged V16 four-stroke unit built by Sulzer in Winterthur, Switzerland. Sulzer’s engines had been utilised very successfully in earlier mainline locomotives, such as the class 45 ‘Peaks’ and the ubiquitous class 47, arguably the most successful British diesel design of the era. These earlier locomotive utilised a twin-crankshaft, parallel bank design however, which dated back to the 1950’s and was both heavy and limited to around 2,500hp. Kestrel’s ‘VEE’ unit used just one common crankshaft, saving weight and making it more compact. British Rail had expressed a need for a locomotive with higher power and greater top speed that was versatile enough to operate both fast passenger services and heavy freight; this was Brush’s response to that requirement. They made the case for a single-engine design of very high power, arguing reduced maintenance costs when compared to twin-engined locomotives such as the class 55 ‘Deltic’, and eliminating the need for double-heading onheavy freight trains. (The main counter to this argument was that if a
twin-engined formation suffered engine failure, it could at least limp home. This proved a decisive factor in BR’s eventual decision to order a fleet of IC125 HST’s for their fast Inter-City services.)
Externally, Kestrel shared slab-sided flanks similar to the class 47, but the cabs were aerodynamically rounded, with deep, twin wrap-round windscreens givi ng a far more modern appearance. It was technically advanced for its day, too, with plug in solid state modules used for voltage and power control, which included wheelslip and engine temperature monitoring.
Hawker-Siddeley made the official handover to British Rail at London Marylebone station in January 1968, and the loco was subsequently put to work, mainly on heavy freight trains, notably including a coal train of over 2000 tons. The main reason for not initially deploying Kestrel on fast passenger services was to prove it’s Achilles heel – the axle-loading. After the Hither Green accident of 1967, (in which 49 passengers died after a Hastings to Charing Cross service derailed due to a broken rail) BR issued a directive that all locomotives should have a maximum axle weight of 21 tons. Despite the weight-saving offered by the prime-mover, Kestrel was still a big, heavy locomotive, weighing in at just shy of 130 tons. Consequently, the axle loading of over 22 tons was deemed incompatible with BR's requirements. This eventually led to the replacement of the bespoke bogies with modified class 47 items and traction motors (which were lighter, but less powerful) by Brush, in an attempt to overcome the weight issue. This resulted in Kestrel being deployed on express trains, principally on the East Coast Main Line on diagrams normally handled by the ‘Deltics’, which it comfortably outpaced, but the axle load issue still lingered, and in 1971 the locomotive was refitted and reunited with the original bogies, then sold to Russia for the princely sum of £127,000. After the sale, the Russian government tested the locomotive heavily and even removed the power unit for static tests but sadly the locomotive never actually ran in service and was stripped of all usable parts and used as ballast to test other locomotives. The locomotive was sold for scrap in 1993 and cut up a year later.