axle-load limit specified by British Rail for its procurement requirements. The locomotive was officially handed over to BR on 29 January 1968 at Marylebone Station. Test runs were performed with both passenger and freight stock; the locomotive was primarily used to haul heavy freight trains - including a coal train of over 2000t weight - the locomotive achieved an 88% availability figure after . Following the fitting of its new bogies the locomotive was used on express passenger trains. On a London King's Cross to Newcastle service diagrammed for British Rail Class 55 ('Deltic') operation the unit was 14mins faster than required even though the new traction motors meant reduced power at the rail. In March 1971 it was withdrawn from service in the UK and sent back to Brush for refitting. Sale to Soviet Union The locomotive was sold to the Soviet Union in 1971 for £127,000, being shipped from Cardiff Docks to Leningrad docks by the MV Krasnokamsk in July 1971. On arrival in Russia, Kestrel was exhibited at the Moscow Rolling Stock Exhibition and then was moved to the All-Union Rail Transport Scientific Research Institute at Shcherbinka where it was re-gauged to , and tested on a circular test railway as well as being used on some parts of the Russian rail network. After testing of the locomotive the engine was removed for static testing, and the locomotive body ballasted for use in high load tests of other vehicles. The remains of the vehicle are believed to have been scrapped in 1993. Conclusions United Kingdom The locomotive did not result in any further orders. Russia Some of the observations of Russian railway engineers relate to different practices between Russian and British locomotive design, specifically: Due to the smaller loading gauges in Great Britain, the cab windows were set too low for signals to be seen if the driver is standing. The passages were too narrow. The bogies were considered overly large. The requirement to sling the engine and compressor below the main frame was considered unusual. The stressed skin construction was noted as being not suitable due to corrosion increasing the susceptibility to structural weakening, as the load-bearing skin is thin and point welded. The axle bearings, traction transmission and spherical rubber metal joints in the suspension were considered of interest. Developments Some of the HS4000's technology was used to form the basis of, or improvements in,