In the early 1970s, the British Railways Board made the decision to replace its main-line express diesel traction. Financial limitations were tight, so mass electrification was not possible. As a result, a new generation of high-speed diesel trains had to be developed.

Experience with the high-speed Class 55 Deltic locomotives had shown that a low axle weight was essential to avoid damage to the track at sustained high speed, and that high-speed engines were the only way to provide a good enough power-to-weight ratio for diesels. To power the HST at up to 125 mph (201 km/h), each power car had a new diesel engine, the 12-cylinder Paxman Valenta, running at 1,500 rpm and developing 2,250 bhp (1,680 kW). The 70-tonne weight of the power car gave it a 17.5-tonne (per-)axle loading. The British Rail Class 43 (HST) is the TOPS classification used for the InterCity 125 High Speed Train (formerly classes 253 and 254) power cars, built by British Rail Engineering Limited from 1975 to 1982, and in service in the UK since 1976.

The class is officially the fastest diesel locomotive in the world, with an absolute maximum speed of 148 mph (238 km/h), and a regular service speed of 125 mph (201 km/h). The record run was led by 43102 and trailed by 43159.

In 1987, as electrification of the East Coast Main Line was under way, British Rail realised that the new Mark 4 carriages for the Class 89 and Class 91 locomotives were not going to be finished in time for the introduction of electric services on the East Coast Main Line so, in late 1987, a total of eight Eastern Region power cars (Numbers 43013/014/065/067/068/080/084/123) conversions (on 43014/123) were carried out at the Derby Engineering Development Unit, whilst the other six (43013/065/067/068/080/084) were converted by the diesel repair shop at Stratford to have the lower valancing removed and buffers fitted.

After being fitted with buffers, these power cars began work as surrogate DVTs to work with the Class 91s and 89. The locomotives, working with conventional Mk3 stock, worked on the line between 1987 and 1991, when the last Class 91 locomotives entered service. As well as buffers being fitted to these powercars, special remote control equipment was also added to the locomotives so they could be controlled by the locomotive at the front. Once these locomotives left DVT duties, the remote equipment was removed.

After privatisation, these power cars joined the Virgin Trains fleet working both Virgin CrossCountry and Virgin Trains West Coast routes, where they displaced loco-hauled stock. All the units were repainted from their original InterCity colours to the Virgin red livery. Later, Virgin Trains withdrew the HSTs when new Class 220 and 221 units were delivered, and nearly all of these power cars went into storage at Long Marston.

After years of storage, several of the powercars were bought by Midland Mainline to be part of Project Rio, special services running from London St Pancras to Manchester while major engineering works were undertaken on the West Coast Main Line. These units were kept in the de-branded Virgin Trains livery throughout their time with Midland Mainline and put back in storage once Project Rio had finished in 2006.

43013 and 43014 joined Network Rail's New Measurement Train in 2003 and have continued to work with this service ever since. Both of these units have now had MTU engines fitted.

43080 was leased to GNER as a one-off powercar, working as a spare unit that could be easily called for if a HST failed. For most of its time with GNER, it was based at Craigentinny yard in Edinburgh and was painted into GNER colours. This locomotive's lease ended in 2006 and it was returned to storage at Long Marston.

In 2007, Grand Central took an interest in the stored power cars and amalgamated them into its fleet of three HST sets. In total, 43065/067/068/080/084/123 were bought by the company and now run high-speed services between Sunderland and London Kings Cross. HSTs 43084 and 43123 were the final operational Paxman Valenta power cars, being re-engined in 2010 with the MTU treatment. While at the works being re-engined, Grand Central added the orange stripe that appears on its Class 180 units, re-painted the front ends (making them look more like the non-buffered HSTs), and re-numbered the power cars into the four-hundreds. These are the current numbers: 43465 (065)/467 (067)/468 (068)/480 (080)/484 (084)/423 (123).

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