The Intercity 125 High Speed Train was introduced in October 1976  following trials earlier in the decade with the prototype HST Class 41/252. Originally the HST was designated a diesel electric multiple unit with units allocated to Western Region numbered Class 253 and those on Eastern Region Class 254.
However in the 1980s the power cars were allocated the TOPS number Class 43 (the original Class 43 was the "Warship" diesel-hydraulic locomotive), they are after all proper locomotives capable of independent operation, the Mark 3 coaches they haul/propel do differ from loco-hauled Mark 3s with different electric systems and a lack of buffers and can only work properly with Class 43s. The Class 43 only has a driving cab at the streamlined end, the prototype Class 41 also has auxiliary cab controls for shunting at the other end.
Refurbished by Brush Traction (2006-9)
|Engine:||(Original) Paxman Valenta 12RP200L diesel
(Refurbished) Paxman 12VP185 or MTU 16V4000 R41R diesels
|Power:||(Valenta) 2, 250 hp (1, 678 kW)
(12VP185) 2, 100 hp (1, 565 kW)
(MTU) 2, 250 hp (1, 676 kW)
Ironically the HST only came about because of a lack of funds for electrification of more main line routes after the completion of the WCML . The HST was devised as a stop-gap... that has so far lasted forty years! Although they have been slowly replaced from some of their original routes they have been cascaded onto other routes and many should continue to serve well into the 2020s. In the mid to late 2000s the fleet was heavily refurbished with the original Paxman engines replaced mostly by MTU units  though some have also been fitted with Paxman VP185 diesels since the early 1990s. Despite forty years and millions of miles of travel only a handful of Class 43s have been withdrawn, and those due to collision damage.
The Class 43 has also been exported to Australia where power cars heavily based on the Class 43s powered the XPT which is still in service . In the late 1980s a small number of Class 43s were taken out of service, fitted with buffers, and formed driving van trailers for the new Class 91 which replaced the HST on the newly electrified East Coast Main Line. Once purpose built DVTs had been built the modified Class 43s were returned to service .
The Class 43 currently serves with GWR, Cross Country, East Midland Trains, Virgin East Coast and Grand Central and has served with a number of other TOCs too in the privatised era. Some cars are also in service with Network Rail in the New Measurement Train. Replacements in the form of the new 8xx series IEPs are being built though the HST will remain a fixture on British rails well into the next decade. Refurbished shorter sets are being used to revitalise Scottish and South West England intercity routes.
|Cross Country 43 384 at Derby|
|GWR 43 002 (repainted in original BR livery) at London Paddington|
|East Midlands Trains 43 055 at Nottingham|
|Cab side view of 43 002|
|GWR 43 185 repainted in BR Intercity livery at London Paddington|
|The New Measurement Train at Sheffield|
 Brian Haresnape, High Speed Trains (Ian Allan, 1983) p. 51
 Colin J. Marsden (ed.), "The Production HSTs", Modern Locomotives Illustrated No. 208 August-September 2014 (The HST Part 1: The BR Days) p. 28
 Colin J. Marsden, Traction Recognition (Second Edition) (Ian Allan, 2009) p. 34
 Marsden, MLI 208 p. 71
 Marsden, Traction p. 36