There was an interesting film about the development of the British Rail, diesel powered, Intercity 125. You can watch it, here
The train was the flagship modernisation project of British Rail at the end of the 1960s. The Intercity 125 replaced the messy old combo of engines and different carriages with a rake of unified rolling stock and with a power unit at each end.
The Intercity 125 was designed by Kenneth Grange. The speeds made possible by the new train transformed the shape of Britain.
Early in the film, Peter Parker, the chairman of BR, makes the point that the railways take a long view. That’s certainly the case with the 125. Many are still in service and some are expected to keep going until 2035!