In-order to allow running at 125 mph on Britain's Victorian era railways, new rolling stock was needed by British Railways. Significant improvements over 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 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. After the HST Mk3 coach variant was introduced, further Mk3 coaches were introduced to the West Coast Mainline for use as part of locomotive hauled trains.

Grand Central Railway purchased six Class 43 power cars and twenty-four Mk3 passenger coaches from Porterbrook Leasing in 2006. Major modifications were required for the Class 43 power cars to make them compatible with the Mk3 coaches. These included changing the train supply from three-phase to 1,000V conventional ETS, fitting jumper connections, changing the central door locking system and more modifications to the couplings and electrical systems. Grand Central launched their new London King’s Cross to Sunderland passenger service on 18 December 2007. 

The Mk3 TSO coach model sports the distinctive Grand Central black and orange striped livery with the iconic Grand Central logo emblazoned in the middle of the stripe. Featuring an accurate corridor connection and close coupling, this model is suitable to run with a Class 43 power and dummy car from the R30375 Grand Central HST Pack.