There was this but not the decade you are thinking of:
The First Train Now Arriving... (1975) (TV)
E.
Possibly might have been a 90's drama documentary on the BBC about the race to build the first and best stream train.
There was this but not the decade you are thinking of:
The First Train Now Arriving... (1975) (TV)
E.
There was also a Timewatch episode, broadcast in 2003:
NRM | Press Office | Press Release
E.
BFI Filmstore £5 off British Transport Films
British Transport Films Collection (18-disc set)
BFI Filmstore British Transport Films
The BBC or ITV made a drama documentary in 1980 about the development of The Rocket and the Rainhill trials to coincide with the 150th anniversary of the Liverpool and Manchester railway. Timewatch covered the story again in 2003