InterCity 125
During the same period, British Rail also invested in a separate, parallel project to design a train based on conventional technology as a stopgap. The Intercity 125, otherwise known as the High-Speed Train (HST), was launched in 1976 with a service speed of 125 mph (201 km/h) and provided the first high-speed rail services in Britain. The HST was diesel-powered, and the Great Western Main Line (GWML) was the first to be modified for the new service. Because the GWML had been built mostly straight, often with four tracks and with a distance of 1 mile (ca. 1.6 km) between distant signal and main signal, it allowed trains to run at 125 mph (201 km/h) with relatively moderate infrastructure investments, compared to other countries in Europe. The Intercity 125 had proven the economic case for high-speed rail, and British Rail was keen to explore further advances.
Read more about this topic: High-speed Rail In The United Kingdom, History