Officially the fastest diesel trains in the world, the InterCity 125 High-Speed Trains were introduced by British Rail in 1976, operating 125mph services on the Great Western Main Line and the East Coast Main Line before being introduced to other lines as their use spread.

When originally built by BREL at Crewe Works the InterCity 125 units, consisting of a pair of Class 43 power units and a rake of Mk3 coaches, were considered to be diesel multiple units and allocated as Class 253 to the Western Region and Class 254 for the Eastern Region, as it was envisaged that the sets would remain in fixed formation.

The Class 43s would begin to see a reduction in their use as electrification became more widespread around the UK, with their services no longer being required on the East Coast Mainline. With the introduction of the Class 800 sets the Class 43 has been relegated to regional services with Scotrail and GWR being the two largest operators of the Class in 2022. Due to the locomotives status as the fastest diesels in the world many examples have been preserved.