To allow locomotives to run at 125mph on Britain's Victorian railways, British Railways needed new rolling stock. Significant improvements of the Mk2 included new secondary air suspension between the bogies and the coach body as well as aerodynamic skirting on the underframe.
Mk3 coaches are 75ft (23m) long, enabling far greater capacity than older coaches. Mk3 coaches also incorporate disk brakes and wheel slip protection for faster deceleration. The first Mk3 coaches to be delivered were used as part of the HST prototype along with the two Class 41 diesel power cars in 1972.
Mk3 coaches entered service in 1975, along with the Class 43, forming the iconic InterCity 125 trainset. After the HST Mk3 coach variant was introduced, further Mk3 coaches were sent to the West Coast Mainline for use as part of locomotive-hauled trains. Whilst Mk3 stock is standard for HST units, the standard locomotive-hauled stock is Mk3a.
Mk3a stock differs from Mk3 with the inclusion of buffers as well as a different electrical system that uses motor generator units in each coach to power air conditioning and other ancillaries. Mk3a stock was built until 1984 before 3B stock, with improved seating and lighting, was built from 1985 to 1988.
The Office for Passenger Rail Franchising (OPRAF) announced that GNER had won the Inter City East Coast franchise in March 1996. Through this, GNER inherited a fleet of InterCity 125 HST sets and rolling stock such as the BR-built Mk3 coaches. The Mk3 coaches were in service from 28th April 1996 when the GNER started operating its new fleet.
In August 1997, GNER acquired twelve Mk3 sleeping coaches for conversion to passenger coaches to make the HSTs longer. The project was not to be, as GNER was able to lease other Mk3 coaches released by Virgin CrossCountry after the operator withdrew its HSTs. By the time GNER’s services were transferred to National Express East Coast on 9th December 2007, there were 56 Mk3 coaches in the fleet.