Under the chairmanship of Stanley Raymond, it was decided to reduce journey times further on long-distance trains by increasing line speed to 125 mph, where practical – the maximum considered possible on Britain's Victorian-age railway.
At the end of 1968, proposals were submitted to the Commercial and Operating Departments of British Rail for a new fleet of third-generation standard coaching stock, designed to run at 125 mph.

The rapid development required for the HST in 1969 made the Mark 3 coach design the obvious choice for this train and, in 1972, the first ten Mark 3 coaches were built for the prototype HST.
Under the chairmanship of Stanley Raymond, it was decided to reduce journey times further on long-distance trains by increasing line speed to 125 mph, where practical – the maximum considered possible on Britain's Victorian-age railway. At the end of 1968, proposals were submitted to the Commercial and Operating Departments of British Rail for a new fleet of third-generation standard coaching stock, designed to run at 125 mph.
The rapid development required for the HST in 1969 made the Mark 3 coach design the obvious choice for this train and, in 1972, the first ten Mark 3 coaches were built for the prototype HST.
British Rail Mark 3