In-order to allow running at 125 mph on Britain's railways, new rolling stock was needed by British Railways in order to replace the aging Mk2s. Significant improvements 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 enabling 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.

Following the FirstGroup's decision to buyout their partner's shares in Great Western Holdings, a decision was made to rebrand the Great Western Trains HST units to First Great Western (FGW). Visually this change involved the fitting of a new vinyl gold strip and colour fading as well as fitting new FGW logos. This livery was relatively short lived, with FGW introducing a variety of new liveries over the next few years which were more in keeping with the First Group's other corporate liveries.