43312 and 43206 form a HST service at Cupar in December 2019. ©Richard Birse
The British Rail Class 43 (HST) is the TOPS classification used for Intercity 125 High Speed Train power cars, built by British Rail for use on express passenger services. The Class 43 is officially the world's fastest diesel locomotive, with an absolute maximum top speed of 148mph, and a regular service speed of 125mph. The class are widely regarded as one of the most iconic and influential locomotive designs and were a tremendous success for British Rail. The class were paired with specfically designed Mk3 coaching stock and initially saw use on both the Western and Eastern regions of BR. Over time, they saw use across the network and thanks to their vastly improved speed and acceleration/ deceleration times, they helped to slash journey times. The HSTs continued strongly into the 21st century and although they have now been removed from their original 'stomping grounds', they still perform a variety of services with an array of operators into the 2020s. Many sets have now been stored, but there have been expressions of interest for their use on other passenger workings or even as part of new freight operations.
(Information provided via Wikipedia)